Sunday, August 5, 2007

SURFACE PLATE


[AirVW] Re: Oil pump leak

Wed, 24 Nov 2004 05:48:06

--- In AirVW@yahoogroups.com, "Steve Chilcott" wrote:
> Bob was correct. When I checked the flatness of the full flow face
> plate I discovered there was a cupped sort of channel across the face

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Well, good for me, then :-)

Of course, I was probably out back sawing up stove wood while the Mechanic-in-Charge of Steve's engine was doing all the work so let's give credit where due, which is to Steve.

But Steve's message offers a nice opportunity for me to preach about a couple of things, one being tools, the other being the need to check every part that goes into your engine.

There's lots of ways to check for flatness. One is to bridge the part with a known-true straight-edge such as the blade of your machinist's combination square then hold the part so you can see if there's any light between it and the straight-edge. For something like the cover-plate on the oil pump, that would serve as a quick & dirty check.

A better check would be to use a surface plate and some Prussian Blue. You smear the blueing onto the plate with your thumb, drop the cover-plate in the mess you just made and give it a light twist. If it's truly flat the blue will transfer to the entire plate.

The only problem here is that if you're only building one engine you probably don't have a surface plate. (Fig 2 shows a 12 x 18 surface plate, about the smallest size suitable for blueprinting a VW crankcase.)

Surface plate is a big piece of cast iron or a thick slab of granite. Cast iron surface plates are kinda rare nowadays bu
t 'way back when every apprentice machinist made their own. Not the casting and curing but the flatting. Learning how to flat a plate lead to teaching the apprentices how to scrape and flat the beds, rails and tables of machine tools, which was a necessary periodic chore back in the days before they used induction-hardened demi-steel for machine tools. (Fig 1 is an 8 x 10 high-precision surface plate used for setting-up cutting tools.)

While cast iron surface plates were inexpensive and not all that difficult to make, they did change their dimensions with the seasons due to the expansion & contraction of the metal.

Granite was more accurate over the long term but flatting a hunk of granite took the task out of the machine shop. Which isn't to say you can't flat granite, just that it isn't a commonly taught skill. Unless you're into optics.


Nowadays you can buy a small surface plate for a very reasonable price but if you're only building one engine there are a number of substitutes that will fill the bill. The one most commonly available is a twelve-inch square of polished granite tile, available from Home Depot. To make it useful, get a piece of well dried 3/4" plywood, seal it up good with thinned varnish and after that cures, bed the tile in a layer of urethane painter's caulk. To ensure the caulking will cure, give the back of the tile a spritz of water before smooshing it into the caulking. Once it cures, the caulk will act as an elastomere, isolating the granite from the wood. (Fig 3. is an inexpensive 9 x 12 surface plate used for flatting, checking valve springs and other low-precision chores.)


Prussian Blue is the other head-scratcher for non-machinists, in that most of you won't know where to get it and will find it rather pricey for just one engine. So use lip stick. Yeah, they always look at me when I say that. Forget it. Go down to the Dollar Store or whatever and look for inexpensive cosmetics. Color isn't important. I generally look for stuff on sale, buy a couple of tubes of whatever.

Prussian Blue (or lip stick) serves as your 'spotting compound' when checking the fit of your valves after being lapped, and of the cylinder barrels in their bores and if the top edge of the barrels is true and the mesh of your cam-gear and lots of other stuff. Lipstick also cleans up nice.

Steve's experience with his leaky pump is a nice example why you can't just bolt parts together and expect to end up with a good engine. The fact it runs is not the definitive test. Virtually any collection of VW parts will run.

(NOTE: Indeed, the most common problem is that novice engine builders are not even aware of them, assuming that if the thing runs at all, it's okay to fly.)

The tricky bit -- and the enormous mass of detail not contained in all those swell manuals telling you how to bolt things together -- is what to do to correct the numerous problems that crop up along the way, such as the drips or a tight bearing (or a loose one), valve gears that don't mesh proper or cock-eyed valve train geometry.

Experienced engine builders have found the best solution is to not let such errors occur. To prevent them, they blueprint all of the parts first as individual components then as sub-assemblies, mating them with their associated parts in as many pre-assembly steps as it takes, correcting errors as they are discovered.

This method is an absolute necessity when you have only one engines-worth of parts to work with.

-R.S.Hoover

Wednesday, August 1, 2007

HEADS 102


This photo, stolen from Jake Raby’s web site (http://www.aircooledtechnology.com) is of a Type IV head that has been treated with the full range of coatings.

The black coating on the head is a thermal dispersant that increases the thermal emissivity of bare aluminum by about 8%.

The grayish-white coating on the combustion chamber, the faces of the valves and the head of the pistons is a ceramic-metallic alloy that is a very poor conductor of heat resulting in a higher BMEP for the same CR and amount of fuel.



BACKGROUND
Before there was Yahoo we had eScribe. That’s where the AirVW Group came into being shortly before the turn of the century. (I think it was 1998.) Then Yahoo bought eScribe, promising to maintain the archive of messages. They lied but that’s the norm when there’s money involved.

Unfortunately for the aviation community the archives contained thousands of VW-related messages of real worth, such as the discussion on Thermal Barrier Coatings.

What follows is when the conversation was resumed under Yahoo. It is not complete but for those new to the subject it offers a hint of what has gone before.

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21 Jan 2003
Ceramic coatings
--- In AirVW@yahoogroups.com, CaptonZap@a... wrote:
>a while back you said you would have a report on the
> ceramic coatings and their effect on engine performance. Anything on that
> front yet?

--------------------------------------------

No.

I've proven to myself that cer-met coatings work. But it has taken me nearly three years to come up with a method of applying the stuff that may be suitable for use by individuals.

The most recent development (or lack of it) was the discovery that while an oven made out of cement-board, using the burner from a defunct water heater, works perfectly well, controlling the thing manually by bobbing about peering through holes at an array of oven thermometers -- does not, in that it simply isn't practical. I saved up for a thermostatically controlled over and it is now on-order. This will hopefully allow me to stuff a coated part into the thing, set the timer and forget about it.

(You can't use your household oven for the more exotic coatings. They contain some seriously lethal poisons that will diffuse into the lining of the oven... and back into your cookies when you're done baking pistons. So you need a spare oven. Since gas is the cheapest source of heat, I made a gas oven using junked parts. But the curing temperature has to be kept within a fairly narrow range [narrower than most domestic ovens allow] with is difficult to do without expensive controls. So I did it manually. And while it worked, it was a major pain in the ass. So I'm trying something else.)

Another reason I'm not ready to post anything on TBC's is because I don't have any quantified data for full-size engines. I've seen some rather remarkable improvements in the one-cylinder lawnmower engines I used for testing but I don't think the data from a 6cid one-lunger can be extrapolated to big-bore VW stroker.

For anyone who isn't familiar with Thermal Barrier Coatings you'll find lots of information on Tech Line's website. TBC's were originally developed to protect the turbine blades in the hot-section of a turbojet engine. And they do. But what's really exciting are the hyper-eutectic coatings. Just as an alloy of lead and tin can be ratio'd to melt at a temperature lower than either of the base metals, it is possible to obtain a unique hybrid coating of ceramic and metal which fuses to the substrate at relatively low temperatures, allowing the stuff to be applied to aluminum engine parts.

In theory, you can build yourself a better engine by simply spraying on some magic stuff then baking it in the oven. In fact, the material has to be extremely clean, the surface to be coated must have a certain texture, the coating must be applied in the right thickness, dried to the right hardness then baked at the right temperature. Some coatings require post-baking procedures. Develop a system for forged mild steel, such a VW crankshaft, and it's liable not to work for cast iron (such as a VW cam shaft) And what works on a cast iron cam shaft may not work on a cast iron cylinder barrel. Most frustrating of all, there is no manual for any of this.

(NOTE: The photo is of a Type IV combustion chamber on one of Jake Raby's race-winning engines.)

Do it right, you end up with a piston & combustion chamber that withstands higher temps and pressures. Or bearing journals that are self-lubricating. Or exhaust valves that shrug off heat. Or an exhaust pipe that runs cool to the touch (!) Or surfaces that radiate 8% more heat. Modern-day magic... once you figure out how to do it :- )

The stuff -- a water-based slurry in most cases -- is fairly expensive. Do it wrong, you end up with a mess. An expensive mess. I haven't spent much in actual dollars -- mebbe five hundred bucks -- but it has eaten up hundreds of hours and all sorts of little jigs and fixtures to hold or rotate a particular part.

(NOTE: $30 for 3 ounces of CBC-1. That’s enough for at least three Volkswagen engines if all of the parts are prepped and can be sprayed at one time. July 2007)

For those of you with the money, there are shops that will apply your coatings for you. But as I said, it's fairly expensive. On the VW heads, for example, you may elect to use a barrier coating on the chamber, valve heads and exhaust port, plus a friction reducing coating on the valve guides & stems, plus a thermal dispersant on the outer fins and the floor of the valve gallery. Unfortunately, none of the shops I've talked to had ever applied thermal dispersants to finned aluminum heads. (And now I know why :-)

(NOTE: As of July 2007 it costs about $450 to have a professional shop do a basic coating job on a VW engine. But not every Tech-Line ‘approved applicator’ does engine coatings. Most are just powder-coating shops that also do a few of Tech-Line’s exhaust system coatings.)

Since the stuff ends up becoming a part of the metal to which it is applied, it is also useful as a surfacant, allowing you to protect parts against corrosion. The self-lubricating properties are especially interesting to gunsmiths and machinists.

-R.S.Hoover
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23 Jan 2003
Re: Coatings and ovens
--- In AirVW@yahoogroups.com, "Jack Hohner " wrote:
> These coatings sound like they would really extend the durability of
> the valves, expecially when combined with Bob's HVX oil flow
> modifications. But it sounds like it might be easier just to do a
> valve job?
> ---------------------------------------

Dear Jack,

You're probably right. And you're not the first to raise this point :-) In fact, it goes quite a bit deeper than that.

None of the things I do to an engine are especially significant of themselves. I spend a couple hours cleaning all the casting flash out of the heads and opening up the air-ways around the exhaust stacks. To the casual observer the heads looks the same. And his engine runs about the same as mine. If he's got his CHT under the spark plug, it will even say his heads are as much as 100 degrees cooler than mine, which picks off CHT nearer the exhaust stack. So his valve gallery is dark with cooked on varnish and he needs a valve job after a hundred hours and I don't. No varnish, no sticking, compression in the 120's and even all around, leak-down a scant 10%.

A hundred hours is a full summer of flying fun and the local VW shop will probably do his valves for $25.

Odds are, he'll never makes any really long flights; not over the ocean nor the desert or the Sierras. For his kind of flying a dune buggy engine makes good sense. To him. Until it lets him down. And after it does, assuming he lives through it, he'll do what most guys do in that situation, damn all Volkswagens out of hand and go to a different engine... if he even continues to fly.

------------------------------

The coatings aren't just thermal barriers. Some are lubricants. Others protect the surface from corrosion, allowing an inexpensive and easily fabricated carbon steel part to work as well stainless.

The oven is actually a fairly minor point. (The cement-board jobbie was not my first effort :-) Getting the surface clean is trickier than it may appear -- most of the coatings are water-based. That means ANY oily residue will give you fish-eyes. The surface texture is also critical to the final finish. Coarse sand works pretty well for aluminum, such as the piston tops and combustion chambers, but you need silicon carbide for the valves. (NOTE: I now use #120 aluminum oxide.) Then comes laying on a uniform coating of whatever material you're trying to apply to that surface. Most of them you can apply with an air brush but getting down into the exhaust port is fairly tricky and air-borne application doesn't work at all on a deeply finned surface.

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Although it isn't entirely correct I tend to think of the modifications to the lubrication system as COOLING enhancements, whereas the tungsten- and molybdenum-based coatings applied to the cam and bearings and crank and valve stems... are the real lubrication enhancements. The thermal barrier coating applied to the pistons and chamber may result in a slight increase in torque. Or they may not. (In a lawn-mower engine the best I can say is that the treated engine did the same amount of work on slightly less fuel.) But applied to the exhaust system TBC's can have a profound effect on getting hot air to the carb and preventing rust.

-----------------------------------------

Spritzing eight valves with TBC then baking them isn't much trouble, not when you're spraying other parts too. But as you've pointed out, it's tough to justify ANY of this stuff -- coatings or modifications - - if a guy only wants to buzz around his south-forty on a warm summer evening.

I gave up arguing the point years ago. The engines can speak for themselves.

-R.S.Hoover
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25 Jan 2003
Re: Coatings and ovens
--- In AirVW@yahoogroups.com, "Jack Hohner " wrote:
>
> Actually, I was hoping that with the full flow oil mods, one might
> expect the valves to hold up for 1000 hours. And then pull the
> heads, find minimal wear and bring them back to new specs. Is this
> unrealistic?
>

Dear Jack,

I think 1000 hours is unrealistic for carbon steel valves with solid 8mm stems. With Type IV heads you can use sodium-filled exhaust valves having 10mm stems. The larger stems are known to have a better wear factor (most airplane engines use valves with 1/2" stems... 13mm in diameter) but I don't know if they could go a thousand hours. --------------------------------------------

> I like the coating mods and probably should do them while I am doing
> the oil flow mods. From the discussion, it sounded like the coatings
> are pretty tricky.
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Some of them have been. For me :-) I haven't seen any real 'discussion' on the application of coatings. In fact, over the last three years I haven't run into anyone else trying to teach themselves how to apply the stuff, other than the usual nay-sayers.

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> Is the friction coating a bit less fussy to apply
> than the TBC? I thought I read some reference along those lines. It
> seems that would help with the valve guide wear, if one is balancing
> return on effort expended.
>

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WSX (tungsten disulfide) is a dry powder that is burnished into an oil-free metal surface. I think this is a good choice for applying inside of things, such as valve guides and the bearing surface of rocker arms, the push-rod 'cup' in the tappets & rockers.

(NOTE: It takes a fairly heavy pressure for the stuff to burnish-in properly.)

I think it's fair to assume that all engines will eventually use coatings on ALL surfaces. A big question for me right now is which coating is best for what surface. And just to make it interesting, about the time you've figured out how to apply a particular stuff, they're liable to introduce something that has better specs. Do you start all over again? (I have, a couple of times.) Or run with what you've got?

(NOTE: My most recent batch of Tech Line’s ‘CBC’ has a different viscosity from the last batch, requiring a smaller nozzle on the spray gun and some minor changes in how the stuff gets applied.)

One problem I have with the coatings is that aside from using them up learning how to use them, once I have a workable receipe I immediately see applications for the procedure. I've just moly- coated my first 10" saw blade, for example. And yes, it cuts smoother.

-R.S.Hoover
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Re: Copper State
(NOTE: Someone proposed I conduct a ‘VW Seminar’ at the Copperstate Fly-in.)
--- In AirVW@yahoogroups.com, Andre Viljoen wrote:
>
> How about it Bob? What's your feeling?
> ---------------------------------------------

Dear Andre (and the Group),

I appreciate the thought but I'm pretty busy right now. And I really haven't anything to say that you can't read for yourself in the manuals from Continental, Lycoming, Pratt-Whitney or Wright.

Later this fall I hope to have some quantified comparitive data from my thermal barrier coating experiments. (Which ISN'T in any of the manuals.) The preliminary work with one-cylinder engines indicates TBC's put an advantageous kink in the BMEP curve for air cooled engines. Extrapolation from 6cid to 140cid offers some evidence that the three years of experiments may pay off. Or they may not.

The truth is, I'm not entirely clear as to the purpose of a 'seminar' on engines since an engine is perfectly capable of speaking for itself. Indeed, over the years I've found it best to let the engines have the final word since they tend to do so anyway :-)

-R.S.Hoover
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14 Oct 2003
Re: Enlarging jug fin area
--- In AirVW@yahoogroups.com, "Nicholas Cafarelli" wrote:
> I have been searching online for a while trying to find out if anyone
> has ever experimented with enlarging VW jug fins.
> -------------------------------------------------

Dear Nick (and the Group),

I've tried this approach. And wrote about it... somewhere.

Clamping doesn't work. This was a trick tried by the motorcycle crowd way back when as a means of squeezing a few more hp out of two- strokers. While it looks good on paper, the clamped-on extensions do not form a good thermal bond with the existing fins but they do a nice job of obstructing the flow of air to them.

I welded extensions to the fins and got excellent heat-flow across the weld. But it was very time consuming and after making the mod, which I called my Fat Fin heads, they would not fit on my existing tooling.

The exhaust stack, especially its 'elbow' right where the valve guide is installed, is the hottest part of the head. You can add a couple of eyebrows to the top, from the diagonal stay between the spark plug hole and the middle stud(s)but you can't go out any farther than two fins (about 5/8") or you'll obstruct the spark plug hole.

Adding more fins doesn't help unless the heat can easily flow INTO the new fin area, and there is good air-flow OVER the new fin area. With Type I heads you run into both of these problems. It does no good to add fins under the exhaust stack since there's no convenient way to get air to them. Welding fins perfendicular to the existing fins, outboard of the exhaust stack, does little good because heat can only get into them through a narrow bridge adjacent to the lower exhaust stud.

You can add a significant amount of fin area to the eight large fins but based on temperature readings, increasing the size of the four fins closest to the crankcase was pretty much a waste of time. More fin area means more air-flow down THRU the fins. Causing more air to flow down through COOL fins didn't help the situation up near the exhaust stack. And you can't extend those fins because OF the exhaust stack.

Take a look at Porsche heads. To get more cooling you have to do something similar to what Porsche has done, which involves moving the exhaust valve and relocating the stack. Or look at the Type IV, which moved the stack to the bottom of the head, thus freeing up the entire end of the casting for cooling air flow.

I really wish this experiment had worked better than it did. It was a lot of work and once modified, the head could not be used on a vehicle since they would no longer fit under the stock shrouding.

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Another approach to the problem sounds almost too easy to be true.

In order to add valve stem seals I have to replace the guides even on brand new heads so I generally punch them out pretty early on in the process, usually right after I've opened up the chambers and cc'd them. after the guides have been removed is the perfect time to clean up the castings down inside the ports. For the exhausts this usually involves the removal of a significant amount of metal, especially if there are any inclusions (ie, core debris). The exhaust ports on a stock head are pretty small -- under 1-1/4" -- and I usually open these up to about 1-3/8" to match the exhaust manifold.

When you get done cleaning things up the surface should be perfectly smooth with about a #600 finish. (According to NASA, a smoother surface will not flow any better.)

In theory, the greater surface of the ported exhaust stack will absorb more heat so it makes good sense (to me) to apply a thermal barrier coating.

If you come up with a good method of applying a TBC to an exhaust port by spraying, please let me know. I tried all sorts of extended nozzles and so forth but finally resorted to painting the stuff on with a foam brush, trimmed down just for that purpose.

Adding fin area allows you to couple more heat to the atmosphere; you should be able to generate MORE heat without seeing the temperature rise. Thermal Barrier Coats slows the adsorption of heat by the surface to which it is applied and MAY allow you to achieve the same goal for much less effort.

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In a related vein, I have received a couple of queries regarding comments made about 'thermisters,' which is my mis-spelling of thermistor. Specifically, the bit about dipping them in heat-sink compound then wadding them into a piece of aluminum foil. The object there was to provide a good thermal path between the fins of the head and the thermistor; you poke the things right down to the bottom of the fin using an ice-cream stick or similar.

Thermistor do NOT read directly in degrees of temperature (unless you're very lucky :-) All they do is alter their resistance according TO the temperature. You must compare their resistance to a calibrated thermometer to make a chart or conversion table. Once you find a thermister having a usable range, buy a batch of them. So long as they are from the same batch, even surplus thermistors are generally close enough so that you only have to calibrate a couple of them to know how the whole batch will read.

Thermistors come in two flavors. Negative types, their resistance FALLS as they get hotter. For direct reading, you generally feed these a voltage and measure that; the hotter the temp, the more voltage you'll see. Positive types, their resistance INCREASES as they get hotter. You can usually read these directly using the ohms- scale of a sensitive multi-meter. But in each case, the numbers on the meter are NOT degrees of temperature; you have to work that out during the calibration process.

The whole idea is that you end up with a reasonably accurate THERMOMETER that's about the size of a grain of rice and can be applied wherever your ingenuity will allow. If you can afford the Good Stuff, Boeing sells some special silver-filled epoxy used to bond thermistors to aluminum. Or you can do like I did, wad them into aluminum foil and clip them to the fins with paper clips (!) or stuff them down between the fins, or fasten them using every other method you can imagine.

You end up with wires running all over the place, hopefully marked according to the LOCATION of the thermistor.

Don't try to get too sophisticated here. YOU become the limiting factor in that you can only make a certain number of readings on each run before things heat up. It would be nice to rig a computer so you could plot temperatures in near real time... Maybe one day. (My test stands already looks like Dr. Frankenstein's basement :-)

-R.S.Hoover
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5 Dec 2003
To All:
I always polish the combustion chamber and top of the piston. I won't be doing that on my next engine because those parts will be treated with a cermet thermal barrier coating.

I always smooth the as-cast portions of the intake & exhaust ports but polish only the exhausts, or in this case, apply a TBC.

Two people have contacted me (over a three month period) asking why I mentioning polishing in one post then apparently reverse myself and refer only to smoothing in another.

The answer is in the context of the posts but since two people have managed to read it wrong I obviously haven't stated the matter clearly enough.

The reason for polishing a surface is to RETAIN THE HEAT OF COMBUSTION. In the chamber, you want the heat to hang around as long as possible so as to develop the highest pressure and the least amount of heat to be absorbed by the piston & combustion chamber walls. Polishing (or thermal barrier coatings) accomplishes that.

Once the exhaust valve opens the exhaust port must respond to both heat and gas-flow. Polishing addresses both issues, in that you want the best possible gas-flow with the LEAST heat-transfer into the head.

On the intake, simply SMOOTHING the as-cast surface is sufficient since the task here is to facilitate the smooth flow of fuel/air mix into the chamber. NACA and Pratt-Whitney have shown there is no improvement in flow beyond a #600 surface finish.

-R.S.Hoover
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11 Nov 2004
Other Things That Work (Valves & cooling)

Thermal barrier coatings. These are hyper-eutectic compounds which form a ceramic-metallic alloy with the base metal at low temperatures but once cured, withstand exhaust gas temps.

When bonded to a surface TBC's reduce the ability of that surface to absorb heat. The material was developed to protect the turbine blades in the hot section of jet engines. By comparison, temperatures in the VW combustion chamber are relatively cool.

Coating the heads & neck-area of the exhaust valves prevents them from picking up so much heat. Coating the exhaust stack prevents the heat from being absorbed by the head when the valve opens. Applying a high-temperature dry-film lubricant to the valve stem & guide promotes better transfer of heat between them by allowing you to run tighter clearances without risk of galling or sticking.

Coating the top of the piston and the combustion chamber prevents those surfaces from absorbing heat as readily, resulting in higher sustained temperature of combustion, giving a higher BMEP for the same compression ratio.

Bottom line: Less heat appears in the heads & oil (it shows up in the exhaust) and a greater amount of torque appears in the crankshaft -- all for the same amount of fuel.

The dry film lubricant is tungsten disulfide and is easy to apply. Simply degrease the part (totally -- boil them in TSP) then RUB the dry powder onto the surface and burnish it in. It will form a molecular bond with the metal. Also works for all of your bearing surfaces and the base metal of your cam & lifters, rocker arms & shafts, push-rod ball-ends and the cam & distributor gearing. The stuff is expensive but just a dab will do ya.

The hyper eutectic coatings with which I've been experimenting produce a ceramic-zirconium alloy... if you apply it correctly. That means proper surface preparation (typically blasting with media to produce a uniformly textured surface) spraying on exactly the right thickness of the WATER-BASED compound, allowing it to dry then baking it at the correct temperature for the required amount of time followed by allowing it to cool in place.

The tricky bit is that some TBC compounds (there's half a dozen of them) do better with some metals than others, and each DEMANDS slightly different procedures in the application, cure, bake and cooling. Mess things up and you get to start over, often with a new part because this stuff don't wanna come off.

The stuff I'm using comes from Tech Line Coatings. They have a web site and they will sell to individuals. But they don't have a lot of air-cooled engine experience so don't expect to find no-fault cook-book type instructions. I've been working with the stuff for about five years now and I've gotten fairly good at it, meaning I don't fuck up as often as I used to, but I'm still a long way from being able to provide how-to info, other than the above. But it's honest stuff, not a pipe dream, and it really does work... when properly applied.

Most engines builders will immediately benefit enormously from using Tech Line's virtually fault-free exhaust system coatings. Main advantage here is that once coated, the durability of mild steel tubing approaches that of stainless steel.

They also offer thick-film lubricants -- stuff that needs to be baked on -- that should be the cat's pajamas for crankshafts... if you've got the balls to use it. I've done just one stock crank with it; I didn't have the guts to try it on an expensive stroker until I've actually seen how it does on a real engine. But it's wizard stuff on saw blades (!) and the like. (Did I mention these are experiments?)

Build your own engines, you become the Mechanic in Charge. That means you sometimes have to try new stuff, which is always a risky business.

Doing the valves, you're pretty safe. Valves aren't very expensive and you probably have a box full of old ones on which to practice. You should have a blasting cabinet and a good air supply. You'll also need an air brush and a variety of jars. (See my article on the subject.) Getting a UNIFORM coating is one of the trickiest parts of the procedure. Heads are a bitch but valves are pretty easy. You need a rack to hold them while you blow on the coating and another rack to hold them while they bake & cure. (If you use the same rack, the TBC welds the valve to the rack.) Once cured, you'll have to cut through the coating when you re-lap the valve. The oven should be electric, accurate and NOT used for cooking. (Some of this stuff is toxic.) Curing temp is typically 300 to 350*F, depending on which coating we're talking about. But forget about using a gas oven unless you can isolate the burner's combustion gases from the object being cured; with some coatings gas heat leaves a slightly stippled surface (probably from the water being produced by the combustion of natural gas (ie, methane) ). The temperature has to be accuately controlled; if the thermostat hunts too much some coatings end up with a surface like an old oil painting.

Doing heads are the trickiest because you're looking at three different coatings & methods. My oven can only hold one head at a time so it's pretty slow-going with more than one engine in the works.

-R.S.Hoover

PS -- Funny (?) Story: I've got one 'coated' engine running. Stock VW bus engine, running on the stand. About 20 hours on it so far running without any load (ie, flywheel; using about 1 gph). It has proven to be an extremely boring engine. No surprises at all. And no symptoms either; it appears to be totally bulletproof.

The funny part is that I did have a couple of problems, which got me all excited, ready to tear that sucker down so I could see the PROOF that this miracle stuff DID NOT WORK.

The first problem proved to be a dirty carb. (The test stand is out in the weather; shit happens.) The second was a bad ignition lead. (Only five or six years old...)

At the present price of gas (My last fill up cost me nearly $100) I'm about to give it up as a bad job, go ahead with the bigger engines, risk my ass with some air under the wheels. But I thought it was funny as hell going to Defcon One and having it turn out to be a bad wire :-) (Is that a MISS? Bad valve? Burned piston? TBC spalling off the combustion chamber? Contaminated plug? Broken adjuster? Bent push-rod?) I was all ready to roll it into the shop, start stripping her down to find out what went WRONG, as is the usual case with most experiments.

But four years and counting, finally got to a real engine (most of the experiments were on lawn mower engines) and the sonofabitch refuses to give me a real problem to work with! (After about 30 hours I'd like to load it up to around 50% and put about a hundred hours on it, then tear it down. But at the present price of gas, that's not going to happen.)

Maybe it's time I started thinking about a vacation :-)
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12 Nov 2004
Re: Other Things That Work (Valves & cooling)
--- In AirVW@yahoogroups.com, "Jack Hohner" wrote:
> Can this stuff be sprayed on
> electrostatically?
----------------------------------------------

I don't know. But I know a good way you can find out :-)

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> Is this stuff sensitive to airflow in the oven.

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I think it's fair to say that it is. I've tried both a rotisserie and a fan, and both together, as a means of evening out the flow during the fluid phase. The fan worked best. Any motion in the part usually resulted in an uneven surface texture.

------------------------------------------------

As for drawing on the experience of others, I certainly tried, including several posts here and to other VW-specific Groups. While there are plenty of NASCAR types using it, all of their applications are for water-cooled engines, mostly on steel parts. Even with aluminum heads, the fact they were water-cooled puts them nearly 300 degrees below the reality of flying Volkswagens.

Lots of good poop about exhaust systems, though. In fact, there's any number of shops that specialize in applying TBC's to exhaust systems. Ditto for pistons. But start talking air-cooled engines and they always get a call on another line :-)

Leonard, the fellow who runs Tech Line, was very helpful but admitted right up front he didn't have a lot of data for air cooled applications and would like to know how my experiments came out. It would have been even friendlier of him to pick up the tab (this stuff is expensive.. for me) but I imagine he hears a lot of BS from wannabee engine builders.

Personally, I'm not comfortable with the system even now; I would prefer to say nothing until I could back it up with several hundred hours of flight. But right now I've no idea when or even if that will be. I chose to mention TBC's because of the thread on oil-cooling the heads, a path I explored nearly twenty years ago and found less than ideal. Ditto for water cooling. Best bet is to start with a completely new casting but that would put the engine out of reach of the typical homebuilder.

I've tried to find the best reliability at the least cost, using methods and procedures that ANYONE could duplicate. If the conversion requires too much machining or too many special parts, we may as well forget the VW as a power plant for grass-roots aviation and start looking at industrial engines and airframes large enough to carry them.

-R.S.Hoover
-------------------------------------------------------

23 Nov 2004
Re: Questions on Thermal Barrier Coatings (Mr. Hoover)
--- In AirVW@yahoogroups.com, "enginegeek2" wrote:
>
> Dear Mr. Hoover,
> I don't know if you have seen this link and what these folks are
> doing: http://www.aircooledtechnology.com/coatings.htm
> ---------------------------------------------

I think it's fair to say Jake and I are on generally friendly terms. But as you can see from the photo of the 2332, there are lubrication mods I do to my engines that Jake doesn't do to his.

(NOTE: If you’ll poke around Jake’s web site you’ll find some nice things he has to say about me with regard to the design of his ‘Down The Middle’ cooling system. It was kind of him to say so publicly but the truth is, most professional engine-builders are known to one another and while our methods vary in minor ways the performance and durability of our engines shows that we’re pretty much singing off the same sheet of music.)

Most of Jake's business is focused on Type IV's whereas I'm still trying to find the best combination of conversion techniques for turning the Type I into a reliable aircraft engine. For example, Jake doesn't like hydraulic lifters whereas they've been standard in small aircraft engines since the 1930's.

One thing the photos do show is that however we mananged to get there, Jake and I appear to have arrived at the same destination. Best example of that is probably the exhaust valve treatment. (Compare the photo to the HVX drawings.)

The photos also show that Jake (or whoever is doing his coatings) appears to have solved a problem that continues to baffle me, to whit, application of thermal-transfer enhancers to the exterior of the heads. (ie, thermal dispersants) The tricky bit (for me) has been to get a uniform coating all the way down into the bottom of the fins. Someone has suggested electrostatics as the best method and they may be right. Otherwise, based solely on their appearance, my home-grown methods appear to be achieving about the same results.

(NOTE: It turned out that electrostatic application was NOT as efficient as applying the coatings in a water-borne medium. See the Tech-Line newsletters.)

The consumer-grade coatings offered by Tech Line, meaning those sold to individuals, are not toxic, unlike some sold by Tech Line for commercial applications.

Although 'kiddie trade' was originally coined to reflect sales of cheap appearance-items to youngsters (ie, mostly imported chrome junk) with the demise of dealer-support for air cooled Volkswagens, the term has come to mean any immature, technologically naive owner of an old VW; age and the price tag have nothing to do with it. For some real examples simply see the rec.autos.makers.vw.aircooled Newsgroup. You will find several examples of individuals, some in their fifties, who have spent incredible amounts on their antique ride -- including a few who have bought Jake's engines -- only to tear them up. Never their fault, of course :-)

-----------------------------------------------------

The whole idea behind my experiments with TBC's was to see if they might be applied to the one-man, one-engine situation we have in homebuilt aviation. My general conclusion is that they can. But I suspect they will not be. Despite a fairly high level of interest in flying Volkswagens, the actual number of builders (or even pilots) turns out to be remarkably small and as a group, do not appear especially adept in the mechanical arts. The idea of setting up some kind of cottage industry to provide coated parts specific to our needs is probably not valid for the same reason: there simply aren't enough of us.

-R.S.Hoover
---------------------------------------------------------

4 Dec 2004
To All:

I dropped a line to Jake Raby asking if he was doing his own coatings.

He's sending the parts out to:
Calico Coatings,
6400 Denver Industrial Park Road,
Denver NC 28037

Telephone, fax and web Tel: +888 236 6079 Fax: +704 483 2149

http://www.calicocoatings.com

Please note, the name of the town is Denver but the State is North Carolina.

I am trying to develop TBC application & baking procedures specific to the VW engine and which any homebuilder can use. This is not a business; I don't have anything to sell and I'm not interested in doing parts for other people.

Those of you who have written to me (or Jake) in this regard should address your queries to Calico.

-R.S.Hoover

Tuesday, July 31, 2007

HEADS 101



(NOTE: This was written in 2003. Use it only for background information. In later posts I'll show the methods & procedures I'm presently using, tell you why some things worked better than others.)


Head Work

Die Grinder is big. Two-handed sort of tool. Home machinists usually have one from Sears, Sioux or Black & Decker. The barrel of the high rpm motor is of a standard diameter so you can use a die grinder holder on your lathe; Po' Boy Tool Post Grinder.

Real die grinders are used by Tool & Die markers to literally sculpt steel. Big die grinders, usually pneumatically powered, sometimes so large they're suspended on a counter-poise. Use one of these, solid carbide burr, you gotta dress for the occasion.

Building a big-bore VW for use in an airplane, there isn't a lot of head work; nothing like what you put into a racing engine. At propeller speeds the flow-rate of even the largest big-bore stroker is small in comparison to something designed to turn seven grand at cruise and peak-out around nine. Still, there is some work to do. Opening up the chambers to accept larger jugs leaves a wide ledge at each end of the valve recess. A flow bench will show that the heads breathe better if the ledge is set back so as to unshroud the valve. Bill Fisher covered this in his 1970-era `How to Hotrod Volkswagen Engines,' which remains in print and is still valid for such things as head work. When you get a copy of Bill's book be sure to study the flow-rate charts. Then sit down and calculate the flow rate for your engine, assuming 100% volumetric efficiency at your designed cruising rpm.

Be prepared to be underwhelmed :-)

Now go back and look at the charts. Notice that your rpm indicates stock single-port heads will do pretty well without any unshrouding or smoothing. Up to you; you're the Mechanic-in-Charge.

I always clean up the heads. Force of habit as much as better performance. The big advantage to this type of work is that the improvement ends up being built right into the engine. Like bigger displacement, better breathing isn't something you have to add on or periodically replace.

Besides unshrouding the valves there's a few sharp edges in the chambers that need to be smoothed. Ever seen air-flow through polarized filters? Comparing the air negotiating a sharp corner to one that has been properly radiused is a real eye-opener when you can see the improvement in the flow. Here again, let Bill's book be your guide. Lotsa good pictures.

Most of the head-work requiring a die grinder is simple smoothing. The head is a casting; the ports have rough surfaces, reflecting the surface of the cores, a lot rougher than the fins and other surfaces which reflect the permanent metal molds used to cast VW heads. (I'll mention those other surfaces in a minute.)

We usta think we got more flow if the passages had a mirror finish. Turns out, according to Pratt-Whitney and NASA, there's no improvement after the surface texture hits about #600. (I didn't believe them, of course. But the flow bench did :-) Why this is true has to do with the fact that fuel/air mixture is not a perfect gas. Flow bench runs straight air unless you dope it with a suspended colloid such as smoke.

Point here is that all you need to do to see a good increase in your in-flow (and thus in your VE) is to get the ports dead smooth. Don't worry about a polished finish.

The way to do that is to start with a flapper wheel or sanding drum in your die grinder and knock down all the casting imperfections. Your hand is your best gauge here. As-cast, the ports feel like rough concrete. Your job is to make them feel like smoothly sanded wood.

Once you've gotten rid of all the peaks and ground out any inclusions and smoothed the trench, you simply shift to a finer abrasive and remove the marks of your first effort. Then do it again. And again.

By the time you've gone down about three graduations in your abrasives (which are also covered in Bill's book, I think... ) the surfaces will be uniformly smooth and have an even, frosted appearance that offers just the slightest hint of tooth to the touch. Go ever everything about three times at that level then shift to your finishing grade, whatever it happens to be. But be warned: Each time you graduate to a smaller size it will take about twice as long to remove the marks from the previous grit. If you've got a die grinder and a box of Cratex hobbs expect to spend about four hours per head.

I didn't have all that stuff when I was a kid. I did my first heads using a quarter-inch drill motor. Worked okay but I think I spent about thirty hours doing a pair of heads; no big deal when you're a kid, right? :-) Nowadays, if I didn't have a die grinder I'd probably shoot myself rather than stand at the bench for thirty hours.

Which brings us to the point of all this.

Some time ago a fellow wrote to ask if he could do a set of heads using a Dremel tool. I told him I didn't know. Recently, two other fellows asked the same question and I felt that justified looking into it.

The answer is a qualified `yes.' It takes quite a bit of time but I found I could unshroud the valves and clean up the sharp edges under the exhaust valve using a hobby-type tool, an inexpensive thing I picked up at Harbor Freight. And you can smooth at least part of the ports. But the tool was too fat to get right into the ports, and it didn't come with hobbs that were long enough to reach. So yes, you can do most of the job, and you should see some improvement in your flow, but not as much as when they were properly smoothed for their full length.

- - - - - - - - - - - - - - - - - - -

As for the fins, I'm sure I've covered this before but I want to keep all of this together, so here it comes again.

You have to remove all of the flash between the fins. I use a pneumatic riffler for this but a sabre saw or even a hand saw will work for most of the fins. Up around the exhaust valves you must ensure several very critical passageways are not blocked. I've posted a drawing of the heads and the passageways are clearly shown but you can see them for yourself the first time you examine a head. Keep in mind that the exhaust stack and the exhaust valve guide are principle sources of heat in this area. The head is designed to have the air flow down thru the head; this is reflected by the drafting ratio in the mold. Air expands as it absorbs heat so the exhaust-side of any cooling air channel tends to have greater volume than the intake-side. Don't upset that ratio or you'll see a pressure drop, meaning the air is not picking up as much heat as before. The pressure differential in all cases should be between six and nine inches of water and this is something you should focus your attention on during your test flights. An airspeed indicator rigged with the pressure port to the upper plenum and the static port to the exhaust area, usually the space forward of the firewall, should show a pressure differential of about 90 miles per hour from inlet to outlet. Or you can rig a barometer or even an altimeter to indicate the pressure differential.

Cooling air pressure differential is not something you want to leave to chance. The Volkswagen engine was designed to use a blower having an output proprotional to the speed of the engine. To properly cool the engine using ram-air you must pay the keenest attention to a host of 'unimportant' details. Not only must the upper plenum provide sufficient pressure, your lower shrouding should provide sufficient containment to force maximum rate of flow through the hottest parts of the head. You can instrument these areas with inexpensive thermistors wired to a single gauge and read via a rotary switch or whatever; something sturdy but temporary.

Adjustments to the system take the form of changing the inlet & exhaust area. I consider the late John Thorpe to be the best authority I've read on cooling horizontally opposed aircraft engines. He wrote a series of articles for `Sport Aviation' or its precursor. See if you can track down his articles. If you can't, perhaps someone can paraphrase them or extract just the equations and post them to the archive.

Once you've cleaned up your fins, seal up the chambers, ports and valve gallery then blast the shit out of the fins with coarse abrasive media at low pressure. What you want to achieve is a rough surface. In fact, blasting the cast fins with coarse media will result in a significant increase in the surface area of the fins.

But don't blast anywhere that will eventually be inside the engine. Abrasive media has a habit of embedding itself in non-ferrous metals, coming loose as the metal goes through heat cycles. Bottom line is that if you don't want abrasive in your bearings, don't allow it to get on the engine to begin with. (Real shops use non-abrasive media for cleaning heads and the like. Walnut shells [which is what I use] or plastic beads. Frangible media such as glass beads comes under the same ban as abrasives.)

(So why is it okay to use abrasive rules for porting & polishing but using abrasive media is evil? Mostly because the ports and chambers are pretty small compared to the valve gallery, but more so because of the nature of the media. Blasted media tends to embed itself whereas the media on a sanding wheel does not. [A 30x glass and a good light will let you answer this question for yourself.])

- - - - - - - - - - - - - - - - - - -

Finally, one of the most difficult lessons to learn with Thermal Barrier Coatings is that the surface must have a roughness equal to #80. The best way I found to achieve this on aluminum is with plain old fashioned silica sand. And yes, it breaks the rules big-time.

(NOTE: Starting in 2005 I began using aluminum oxide abrasive rather than sand. It is more dangerous to the engine but less so to the person doing the blasting.)

To coat the tops of the pistons I was able to mask them off pretty well. After blasting I ran them through the ultrasonic cleaner then a zero-tolerance degreaser and finally into the spray booth for spraying with the TBC. They get cured in a 350 degree (F) oven and are allowed to cool in the oven for 24 hours or until hell freezes over, which is how long it seems when you're dancing around waiting to see if you've just fucked up $200 worth of pistons. Which I did, more than once, except they were gimmes; only pistons from out of the junk box. And you can remove a bad coating by blasting... but don't expect it to come out evenly. Blasting off the bad coating then taking a clean-up cut of about .0015 works. Indeed, you end up with a mirror bright beauty... which you must then carry over to the blasting cabinet and hose to a dull, frosted surface. Life is strange in the engine room :-)

The beautifully smoothed heads got the same treatment. I use solid copper head gaskets on heads that have been bored for larger jugs and you don't want the TBC to be UNDER the gasket, which means masking it off. If you can. I tried several methods. I wasn't entirely satisfied with any of them. Masking tape didn't work; after being stripped away (TBC dried but not baked) there was enough residue from the adhesive to contaminate the TBC. I ended up making aluminum rings about five thou wider than the copper gaskets and swaging them into place as a mask. The second time I did it I remembered to provide some means of pry them out of the chambers without scratching the sprayed TBC :-)

The piston tops, combustion chambers and exhaust stacks received the basic Thermal Barrier Coating. Because of its hyper-eutectic nature, baking at 350F cascades a melting process that results in ceramic- metallic alloy bonded to the aluminum substrate. Because of its ceramic nature, the surface pretty much ignores heat.

How does it work on a full-size engine? I don't know. But soon will.

The reason I mentioned it here is because of the violation of the `no abrasives' rule. Silica is definitely an abrasive. But if there was any abrasive residues left on the surface, they are now encapsulated in a cer-met alloy that you literally can not chip with a ball peen hammer. (I've spent three years convincing myself this stuff is worth the effort. I really wanted that shit to fail... save me all the trouble early on. I still don't know. But I'm starting to lean toward `Hopeful' on the self-delusion meter :-)

What's all this for? The main goal is to extend the life of the valves through better management of their heat load. I might see some improvement in power output because of a slightly higher BMEP. Or I might not. The folks who make the coatings don't have a lot of data on air-cooled engines and liquid cooled's don't even come close to the problems we have.

-R.S.Hoover

Sunday, July 29, 2007

Crankcase Fasteners

A couple of times of year, usually in the summer, I receive a message from an angry young man trying to dismantle a Volkswagen engine. In many cases he has resorted to chisels or screwdrivers; in one case a wedge for splitting firewood was used.

The engine is junk, of course. Which is okay because it is a stupid engine anyway (he sez).

Amazingly, in a few cases it is a Group Letter. The Mechanic-in-Charge has sought advice from a gaggle of friends, all of whom agree there is something wrong with this crankcase.

What's wrong is that the fellow has failed to remove all of the fasteners. Or they have left the oil pump in place. Or perhaps the sump plate. But nine times out of ten, suggesting that might be the cause earns me a nasty-gram, often larded with profanity.

I add the fellow to the Kill File and get on with my life.

I'm sure no one reading this has ever forgotten to remove the oil pump before trying to split the case. And I'm sure no one has ever overlooked the stud tucked under the #1 cam bearing. But you may want to print this out, just in case you ever run into someone with a seriously stuck crankcase.

Be sure to mention that they must remove both the oil-pump cover and the oil-pump itself. If they don't have a pump puller then they may elect to remove the oil-pump studs, or at least two of them, so long as they come from the same side of the crankcase. The same applies to the sump plate in that both the sump plate and the oil strainer must be removed.

You will note there is one stud anchored in the right-hand case-half. (With Volkswagens orientation is always relative to the vehicle. That is, the front of a VW engine is where the flywheel is attached; the fan pulley is on the rear of the engine. In the same vein, the #1 Main Bearing is the one nearest the flywheel. Ditto for the cam bearings. These are the conventions established by the designer of the engine seventy-five years ago and apply to the thirty-million or so Volkswagen engines manufactured since. ) So make sure they haven't overlooked the odd right-hand stud tucked away behind the distributor.

Then there are the three bolts. Sometimes the engine is so clotted with dirt and oil that the lower bolt nearest the flywheel gets overlooked. Have them dig around until they've found and removed all three.

The six M12 studs are difficult to miss but sometimes they fail to remove the washers. In rare cases a washer can jam the threads and hold the case halves as if they were still bolted together. So have them lay-out the six large nuts with their large, thick washers.

Then there are the twelve nuts & washers for the remaining studs, including the odd-ball righty. The one most often over-looked is the one below the #1 cam bearing, which is often completely concealed beneath twenty years of poor maintenance. Make sure they chip away the grunge and remove the hidden nut and its washer.

If the fasteners are neatly arranged on a piece of newspaper it makes it easier to tell if you've found them all. Another good check-off is to physically touch the location of each stud as you count them off.

Once all of the fasteners have been removed along with the pump and sump-plate I prefer to split the case using only the strength of my hands. This isn't as difficult as it sounds, assuming the case isn't fitted with shuffle-pins. Simply grasp the opposite ends of the case at the opposing 'corners.' That is, one hand is positioned near the upper tranny flange, the other under the oil pump. Volkswagen has provided pads in those areas to allow the case to be 'started' with a rubber mallet but if you fiddle with it you'll see you can bring your thumbs to bear against the pads while your hand bears on the other half of the case. By applying pressure alternately you can 'walk' the case apart.

-R.S.Hoover

(Note: I've uploaded the drawing above to the engine archive on the Chuggers Group. The drawing is in it's native format [ie, DeltaCAD] meaning it may be manipulated as required. The drawing isn't especially accurate but it's more than a guess :-)

Friday, July 27, 2007

Basic Jugs - IV 1/2

Several people wondered why I failed to mention checking the fit of the piston rings in their grooves. So I went back and mentioned it. Now allow me to offer you a whiff of Mechanical History.

Back in the Good Ol' Days, which really weren't, the Ring & Valve Job was the staple source of income for the majority of mechanics. Thanks to inadequate air cleaners and no oil filtration at all, somewhere between 20,000 and 40,000 miles of service the engine of every automobile became so worn that the compression would fall so low that the engine could not be started. And if you could get it running, it was liable to burn as much oil as gas. That's when you'd deliver Ol' Betsy up to the local mechanic who perform the Ring & Valve Ritual.

Most people don't know it but the primary sealing surface needed to ensure good compression is not the fit of the piston ring to the wall of the cylinder but the fit between the ring and the top and bottom of its groove. Pulling the pistons and honing the walls of the cylinders was part of the classic Ring & Valve job but the critical work took place in the back of the shop, where the pistons were chucked into a lathe and their grooves re-machined so as to return their upper and lower surfaces to truth, meaning perfectly perpendicular to the axis of the piston. Doing so also widened the groove, requiring the fitting of wider rings, a stock of which was usually kept on-hand. A good machinist could 'overhaul' a piston so that the grooves were a nice match for the next available over-size ring but when they missed they simply honed-down the ring to match the groove, plus the usual tolerance of one to three thousandths. There were machines that could hone four rings at a time but small rural shops usually relied upon boy-power, a surface plate, and a sheet of fine sand-paper flooded with kerosene. The boy's reward was being allowed to test-drive the 'tight' engine around the block a time or two. (Yes, that's me behind the wheel of the Model A, out behind Leduc Motors, the Ford dealer in Turlock, California.)

Things are a bit different today :-) Nowadays, when you buy a set of replacement Pistons & Cylinders for your Volkswagen engine the fact the pistons are already installed in the barrels is good evidence that the rings fit their grooves. They might be a tad too loose but you know they can't be too tight, otherwise they wouldn't be able to compress the rings enough for the piston to fit in the barrel. This reflects the fact that mass-produced replacement parts are manufactured to a looser standard - - a wider range of tolerance - - than the factory-produced original.

If you were building an engine meant to run hours at a time above 5,000 rpm you're probably using forged pistons fitted with high rpm rings. That's when you concern yourself with the precise fit of the rings in their grooves. (Indeed, you might even buy blank forgings and machine the grooves yourself.) But for low rpm applications, such as spinning a propeller, you generally don't. Other than to ensure the ring moves freely in its groove there is little to be gained in measuring the precise amount of clearance because in the practical sense there isn't anything you can do about it other than to order a new set of P&C's... with the strong probability they will be no better.

When assembling an engine from a collection of after-market parts there are many areas in which the close attention of the assembler can pay a significant dividend in terms of power, durability or efficiency. But this isn't one of them.

-R.S.Hoover

Wednesday, July 25, 2007

Basic Jugs - IV



Due to their higher operating temperatures, in air cooled engines the piston ring gap is wider than on liquid-cooled engines. For after-market Volkswagen cylinders you want a minimum ring gap equal to 0.0045" per inch of bore. The jugs shown in these articles have a bore of 94mm - - about 3.7" in diameter. 3.7 x .0045 = .01665. Maximum would be about .005" per inch of bore or about .0185" but you could go as large as .006" per inch - - about .022" - - before you began to see a decline in performance.

What cannot be allowed is a gap that is too small. That is, our negative tolerance is zero, meaning the gap can’t be any smaller than our nominal .017" although our positive tolerance could be as much as .005". Written out it would look like this:

RING GAP = 0.017, -0 / +.005

Select a cylinder. Wipe the bore dry using a paper tower (*). Find the baggie matching that cylinder’s number. Lay-out the piston rings and wipe them dry. Starting with the Upper Compression ring, gently stone the edges of its gap. Use your 3x loupe to inspect your work. It should take only two or three light passes of the stone to break the sharp edges of the gap. Repeat this procedure for all four rings for that particular cylinder. After stoning, wipe the edges with a clean paper towel.

Select the Upper Compression ring. Insert it into the top of the cylinder’s bore. Using a clean piston, press the ring into the bore for a distance of about 1". Use feeler gauges to determine the gap.

Remove the ring, turn the cylinder up-side down and repeat the procedure on the bottom of the cylinder’s bore. The gap should be identical, indicating the bore is not tapered; that it is a true cylinder.

Repeat this procedure for the Second Compression ring. Since we now know the bore is true there’s no need to check the ring at the lower end of the barrel.

The Oil Scraper Rings have an entirely different mission than the compression rings. These rings are meant to provide a compliant contact with the cylinder wall and are a loose fit in the barrel. They will be pressed into contact with the cylinder wall by being installed atop the corrugated band. They require the same minimum gap as the other rings but their upper limit may be as wide as .030" and they would still function effectively. Check these rings in the bottom of the bore, pressed in about one-half of an inch.

Repeat this procedure for all four cylinders.

If the ring gap for either of the compression rings is too large you must order & fit a new set of rings.

If the ring gap for any of the rings is too small you must file the gap wider, being sure to stone the edges after filing.

If you need to widen a ring-gap the most common means is through the use of a special tool designed for that purpose called, appropriately enough, a piston ring filer.


As the pictures show, piston ring filers are widely available. Given the wild diversity of designs I'll leave you to decide which is most suitable for your needs.

As a point of interest I have a ring filer out in the shop, somewhere. I can’t remember when I bought it but I recall it cost nearly three dollars. I still use it for little piston rings but for everything else, I use a plain old-fashioned file. (Hint: Clamp the file in a vise and draw the ring toward you. It would be wise to practice on a couple of junk rings first.)

- - - - - - - - - - - - - - - - - -

In checking the rings, all passed a .018" feeler gauge save one, which was a tad tight. I touched it up with the file and re-dressed the edges. As engines go, this one is proving to be a bit less trouble than most (furiously knocking on wood :-)

I'll be applying a cermet Thermal Barrier Coating to the jugs so the next step is blasting them with abrasive media. To prevent damage to the ring lands and make clean-up a little easier, the body of the piston gets masked-off, leaving only the crown exposed.



After you’ve dressed & checked the rings, if you are not going to apply a thermal barrier coating you may go ahead and re-install the rings to the pistons. Remember that on the oil control ring the corrugated band is installed first, then the two scrapers, one on the upper edge of the band, the other on the lower. As for the compression rings, be sure to check their orientation.

-R.S.Hoover

Tuesday, July 24, 2007

Basic Jugs - III


In manufacturing a cylinder barrel the cast iron blanks are first bored with a multi-point tool then brought to their finished size with an abrasive hone. The last step is to scour the honed bore with a coarser, reciprocating hone operated at a fairly low speed. This produces the characteristic cross-hatching, the grooves of which hold a significant quantity of oil that facilitates the process of breaking-in the freshly assembled engine.

The grooves also hold a residue of carborundum, stripped from the coarse hone. These particles can produce vertical scoring on start-up, creating wounds in the cylinder wall that grow progressively worse over time. Although there are a number of high-tek ways to remove the residue (ultra-sound is one) but the cost of doing so can push the price of a set P&C's right out of the market-place.

Fortunately, there is an effective low-tek method developed in the early days of automotive maintenance. You simply scrub the bores of your newly honed cylinder with an abrasive cleanser. For the last seventy years or so Bon-Ami cleanser has been the preferred stuff but other cleansers containing pumice, chalk or diatomatious earth work equally well. These relatively mild abrasives break-down under pressure and are no threat to the innards of your engine. Alas, you can’t use any of the modern-day scouring powders which often contain such lovely stuff as powdered glass and chlorine bleach. Chlorine is about the last thing you want anywhere near cast iron and powdered glass, while it does a beautiful job of removing the porcelain from the kitchen sink, is almost as bad for your engine as carborundum.

There are two schools of thought on how to scrub your jugs. One sez the only proper way to do it is up & down, the way God intended. But there’s a few heathens who insist on doing it roundy-round, especially those who advocate the use of Lava soap rather than Bon-Ami. Others use Boraxo Powdered Hand Soap; a few mix their own formulations.

Personally, I’ve not noticed any difference at the finish line. In fact, the main difference is between those who do scrub their jugs versus those who don’t. The former make up a lot of the familiar faces in finish-line photos while the latter are rarely seen at all. Some (including me) will argue it isn’t the scrubbing but the overall attention to detail that is the key to a properly assembled engine. Scrubbing your jugs is just another of the many ‘unimportant’ details the newbies joke about and never do since it serves no purpose. According to them.

As to how to give your barrels a bath, the holes for the head-stays divides the barrel into quadrants. Put the barrel into your wash bucket, tub or whatever, submerged in water. Dampen your sponge, charge it with a couple of squeezes of Bon-Ami, 20-Mule Team or whatever, pick up the jug in one hand, the sponge in the other and give one quadrant twenty strokes. Dip, re-charge the sponge, rotate to the next quadrant and repeat. After doing all four, rinse the barrel and the sponge... and do it all over again. Four more times. Fig 1 shows the area set up for scrubbing jugs.

If you’re a Lava Man, same routine except you’re going roundy-round whilst everyone else is doing it up & down.

Expect it to take fifteen to twenty minutes per jug.

But before getting all wet & sweaty go find a big pot, fill it with water and set it to boil. If you’ve got a stove in your shop (I do!) things are a bit easier than if you have to work in the kitchen. Or the back yard. Working outside, the best boiler is probably a barbeque. And yes, you want the water boiling, or as close to it as you can get at your elevation.

You’ll also need a piece of stiff wire to fish the jugs out of the boiling water and a pad of newspaper or cardboard to sit them on after they are sprayed. As in WD-40. Because if you aren’t standing there Johnny-on-the-Spot with your can of WD-40 at the ready, your jugs are rust right before your eyes. And yes, WD-40 is okay for this job. In fact, that’s what the ‘WD’ stands for: Water Dispersant, formulation #40. Developed for Convair back in their Atlas missile days.

Ready to scrub? Then go to it.

After scrubbing a jug, take it over to the hot pot, hook the wire through a hole and slosh it in the boiling water. Do a good job of it; you want that jug to get hot. While it’s getting hot you’re grabbing a rag to use to hold on to the can of WD-40 (soapy hands, etc.).

Raise the hot, rinsed jug out of the water, orient it so the over-spray won’t kill anything and soak it down with WD-40. Let it drip a bit then sit on the drain pad, recover your wire and get busy with the next one.

Figure on spending an hour or more per set of jugs. And that doesn’t count the preparation & clean-up. (Hint: Doing more than one set of jugs at a time will reduce your overhead.) (Double Hint: Add another notch to the fin showing which set the jug belongs to.)

Despite conventional wisdom WD-40 is not a protective coating. It was - - and is - - a water dispersant and while handy for other things, protecting bare metal isn’t one of them. So make up a pad of paper toweling, soak it with motor oil and wipe down the bores of your scrubbed jugs. Careful! The last one out of the pot will be too hot too touch. Try doing a bit of clean-up first; give it a chance to cool down. Okay; now wipe them down and put them back into the box according to their notches/numbers. (Fig 3 up at the start of the article shows the scrubbed jugs cooling in the shade.)

-R.S.Hoover

PS -- Some one familiar with my shop wondered why most of the photos were taken in the patio.

I've got a nice shop with a lot of tools. A nice private shop. In this engine assembly series I'm sticking to basic methods that don't need a lot of tools. I no longer offer engines for sale. I don't encourage visitors, and there are things in my shop I prefer to share only with family & friends.

This will not effect the quality of the engine in any way. Indeed, the pictures are probably a more realistic representation of what the average builder is doing. -- rsh
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