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Old May 24th, 2018, 09:53   #16
Franko6
 
Join Date: May 2005
Location: Sw Missouri
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486,

Your .004" piston-wall clearance is at the VW maximum spec for clearance.

We have imported sets of 79.51mm rings that are intended for an Audi. The rings are .07mm(.00275") larger than our ALH/ AHU STD piston and rings, which are 79.44mm. That would cut the ring end gap clearance to a nominal .00125", or virtually EXACTLY the piston bore fit we aim for. We recommend end gap clearances of no less than .016", which you may have to slightly file the 79.51mm rings for a 'no blowby' fit.

If the job required opening up the cylinder more than what the 79.51 rings can do, we have the .25mm oversize rings, which will carry you to .010" of clearance between stock piston size and cylinder bore. Our coating will then, eliminate the piston slap.


When we work up a set of pistons, we not only clean the piston, but also balance weigh pairs, double check rod dimensions and professionally mark the pistons and rods, then install the rings. We need to know exactly how big the bore is.
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Old June 15th, 2018, 12:07   #17
Franko6
 
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It's always amazing to me, when we come up with new and innovative improvements for piston fit and efficient methods of engine overhaul...

Crickets.
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Lockwood, MO 65682
417-232-4634
FranksTDIs@sbcglobal.net

'02 80k grey leather, 99.5 R.I.P 153k
'85A2 NA 375k, '91 A3 290k Always Silver, Always a Jetta

Last edited by Franko6; June 17th, 2018 at 12:50.
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Old June 15th, 2018, 18:45   #18
STDOUBT
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Quote:
Originally Posted by Franko6 View Post
It's always amazing when we come up with in and innovative improvements for piston fit and efficient methods of engine overhaul...

Crickets.
Amazing? Yes, sir, that actually goes without saying lol.
Could be your audience is stunned into awed silence
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Old June 15th, 2018, 18:48   #19
[486]
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It is interesting that you're using the coatings to make up for large clearance, sort of an elephant in the room kinda thing now that I think of it, knew about the coatings being available both for the skirt and for the crown, but never put the two together into using them to build up undersize pistons.

Only trouble I can picture is that I usually see the skirt coating worn down to aluminum on overhaul, though that might just be a lower grade coating meant for additional lubrication rather than buildup.

On mine, I'm just running them loose on purpose so I can do stupid things with less worries about thermal expansion.
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Old June 15th, 2018, 20:03   #20
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I don’t know Frank06 or anything about him; but I am very pleased with his stage 2 alh cam. I was worried it might be too much, not the case at all. It’s not radical, basically it’s like stock but just pulls harder. Nice work Frank.
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Old June 16th, 2018, 06:15   #21
Franko6
 
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486,

The black skirt coating seen on OEM and just about every aftermarket piston we use, is a graphite wear coating. On some of the pistons, there is an oblong 'hole' in the coating that we don't agree with as for location, but it is designed as a measuring point for the piston skirt diameter, excluding the coating. The coating from the factory is usually .0015" - .002" increase in diameter. The entire purpose of the coating is to wear in for a 'more perfect fit'. As many pistons as we have seen, I can tell you that the wear coating usually has a wear pattern in the ALH engine, for example, that the very center of the skirt coating will wear a circle dead-center in the coating. The older AHU pistons are very similar in construction except there are no brass wrist pin bushings. The AHU pistons seem to continuously wear out the entire coating. I think the brass pin bushing has much to do with this patterning.

As for thermal dynamics of piston expansion, it's supposed to be a science, but I think it ends up being an art. To figure out the expansion properties just for a piston whose combustion bowl is off-center is a problem to be solved all in itself, let alone the heating of each cylinder wall and it's distortion due to less than perfect water flow. We can't possibly include enough information to correct every anomaly in this engine, so we build in compensation factors. That is the skirt coating as it is applied in our most commonly used Nural pistons. We simply expand on the coatings purpose.

I understand running a 'loose' piston to avoid the extreme heat cycling that can happen with a highly modified engine. But 'loose' is relative and the piston slap is wear to piston and cylinder wall. The tighter the fit, the less the piston rocks, the more accurately the rings seat, the longer the whole thing holds together...as long as it doesn't seize. I would say that below the threshold and limit of what the piston can do, below the point of seizing, there are control factors we can incorporate to improve overall performance. The wear coating as designed in the OEM pistons is a 'fudge factor' to allow for wear on a piston as it breaks in for a 'perfect fit'. The whole point of the graphite polymer; although it is soft, and very quickly wears away in an interference fit situation, it is also very slick, with a very low coefficient of drag.

A properly designed coating not only will support the improved fit, but will actually increase oil retention on the skirt, as the coating is not perfectly smooth, but has peaks and valleys. If you have kept up with some of the innovative cylinder wall finishing, like laser honing, the purpose is to remove the peaks and use the valleys for oil retention. There is a quantum improvement in break-in period and effectiveness with both laser and graphite polymer technologies.

The question is does it last? Based on our large sampling of blocks, we know that the original coatings do not completely wear off, but are performing exactly their purpose.

In our perspective, and although the company we are using, claims survivability of coatings up to .020" thick, or .5mm, that amounts to second oversize. We are not doing that. Our limit is to the first oversize; .010" or .25mm. Most of the coatings we are having applied are in the .003" -.006" range. This is not a 'single coating' system. If a thicker coating is necessary, a harder, less abradable substrate coating is applied.
There are two oversize ring sets we use to fit for end gap. One set is 79.51mm and the other is 79.75mm. The first ring set we have found that honing an out-of-round bore usually takes us to a very close ring end gap, without filing.

Here is the cool trick. We have done 'in frame' rebuilds, using the original pistons to revitalize engines for a savings for engine removal and piston replacement. It's a huge savings in time and money and the cost is 1/2 the price of a new set of pistons.

I am really enjoying the new technologies that exploding onto the scene.

Dieselmonkey02,

It's actually Franko6; not Frank Zero Six. But thank you for the compliment. The Stage II ALH and AHU cams are the 'most under-advertised' of our products. The cam has wide acceptability and we have engines up to 240hp using it. We really don't push them and the biggest advertisers are the people who own them. Thank you for your unsolicited response.

We have innovated for several items.

Our Molnar 'H-beam' connecting rods are the other big development we created. There are several innovations in the Molnar rods, but that is another story. It must be a good product. Two of our competitors have tried to go around us to get them. No Can Do.
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1007 Olive St.
Lockwood, MO 65682
417-232-4634
FranksTDIs@sbcglobal.net

'02 80k grey leather, 99.5 R.I.P 153k
'85A2 NA 375k, '91 A3 290k Always Silver, Always a Jetta
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Old June 16th, 2018, 11:54   #22
[486]
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Quote:
Originally Posted by Franko6 View Post
As many pistons as we have seen, I can tell you that the wear coating usually has a wear pattern in the ALH engine, for example, that the very center of the skirt coating will wear a circle dead-center in the coating. The older AHU pistons are very similar in construction except there are no brass wrist pin bushings. The AHU pistons seem to continuously wear out the entire coating. I think the brass pin bushing has much to do with this patterning.
The wear pattern is what tips me off to bent rods. Being a tremendous hack, I don't actually measure connecting rods, just look for wear patterns on the skirt and rod bearing. so long as the engine hasn't seen any damage shortly before disassembly these wear patterns will show any bend.
Very interesting on the difference made by the brass inclusions. Differential expansion is a powerful force.
Quote:
Originally Posted by Franko6 View Post
As for thermal dynamics of piston expansion, it's supposed to be a science, but I think it ends up being an art. To figure out the expansion properties just for a piston whose combustion bowl is off-center is a problem to be solved all in itself, let alone the heating of each cylinder wall and it's distortion due to less than perfect water flow. We can't possibly include enough information to correct every anomaly in this engine, so we build in compensation factors. That is the skirt coating as it is applied in our most commonly used Nural pistons. We simply expand on the coatings purpose.
I understand running a 'loose' piston to avoid the extreme heat cycling that can happen with a highly modified engine. But 'loose' is relative and the piston slap is wear to piston and cylinder wall. The tighter the fit, the less the piston rocks, the more accurately the rings seat, the longer the whole thing holds together...as long as it doesn't seize. I would say that below the threshold and limit of what the piston can do, below the point of seizing, there are control factors we can incorporate to improve overall performance. The wear coating as designed in the OEM pistons is a 'fudge factor' to allow for wear on a piston as it breaks in for a 'perfect fit'. The whole point of the graphite polymer; although it is soft, and very quickly wears away in an interference fit situation, it is also very slick, with a very low coefficient of drag.
A properly designed coating not only will support the improved fit, but will actually increase oil retention on the skirt, as the coating is not perfectly smooth, but has peaks and valleys. If you have kept up with some of the innovative cylinder wall finishing, like laser honing, the purpose is to remove the peaks and use the valleys for oil retention. There is a quantum improvement in break-in period and effectiveness with both laser and graphite polymer technologies.
Add to the offset bowl an oil cooling channel that may or may not be getting oil and you get a lot of chance for uneven expansion. Everyone tries their best to line up the nozzles, but they are in a pretty tricky spot to get perfect.
I really like the idea of mahle's monotherm pistons, being steel they don't expand nearly as harshly as aluminum. Pullers report that they tried to use 'normal' clearance (which to anyone else is slap worn out) and it would just beat the iron cylinder walls ragged, so they run them tight now. Too bad the offset bowl makes the friction welding method currently used impractical for us 8v owners, add on needing to make large runs to be reasonably priced and it's right out the window. I'm holding hope for further advancements in metal printing (which seem very close!) to make some steel pistons for oddball small TDIs.
So it seems the skirt coating is sort of like brake shoes. They wear quickly until making full contact with the drum, then wear almost stops completely.
I may well have to look into the skirt coating when I build another nutso engine, sounds like a very good solution.
Quote:
Originally Posted by Franko6 View Post
Here is the cool trick. We have done 'in frame' rebuilds, using the original pistons to revitalize engines for a savings for engine removal and piston replacement. It's a huge savings in time and money and the cost is 1/2 the price of a new set of pistons.
I am really enjoying the new technologies that exploding onto the scene.
I love seeing things like this, hate throwing away perfectly reconditionable parts. Just like spray welding the seal journals on industrial shafts. Do you use a portable rigid hone such as lisle's #15000? Old guys used to swear by such a hone for "proper" in-frames, and I use one, myself. Doesn't do quite the magic of the factory applied laser finishing though.
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Old June 16th, 2018, 18:34   #23
Dieselmonkey02
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My apologies for the name discrepancy.
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Old June 17th, 2018, 08:32   #24
Franko6
 
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Dieselmonkey02, no problem, unless you are looking for Frank06 and can't find him...

486, we really do like the Mahle pistons, but don't be looking for them to make the ALH style piston anytime soon. When installing the steel pistons, and especially them not being forged, we would go with a very tight tolerance of .0005", as piston and cylinder wall expansion rate coincide. There is a possible 'mushroom piston' on the horizon for a small run. We will see what happens. The skirt coating is still a very good idea with a steel piston.

On another note, we really dislike the mis-advertisement from a competitor who brags about 'forged' pistons. The problem with a forged piston is that the clearances must be increased to compensate for the annealing of a forged piston. They will 'relax' and expand over the course of break-in and become larger. I don't use forged pistons and don't expect to. For the most part, people bandy around terms like 'billet' and 'forged' without knowing what they are talking about.
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1007 Olive St.
Lockwood, MO 65682
417-232-4634
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'02 80k grey leather, 99.5 R.I.P 153k
'85A2 NA 375k, '91 A3 290k Always Silver, Always a Jetta
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Old June 17th, 2018, 09:01   #25
[486]
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The steel pistons do still heat up faster than the block so the expansion doesn't exactly coincide, but the steel expands about half as much (inch per inch per degree) as aluminum does.

you can't do a forging with the oil channels, either
With guys making 1k-1200 hp on stock cast pistons in gen3 GM v8s I think the day of forged pistons being more durable isn't really accurate at this point. Casting technology isn't some guy with a ladle ramming up and pouring greensand moulds any more.
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Old June 17th, 2018, 13:26   #26
Franko6
 
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I'm not so sure what the coefficients, or how much the pistons grow, but there is still a large following for forged pistons. It is a problem, as the pistons are not stable as purchased.

The Lisle is a good deal and great for the money. We use an old Sunnen ($$$!) that I'm thinking Lisle copied the design. We built a depth stop for it to keep the stones out of the oil squirts. It's still a bit of a problem. It can twist your arm off. Work it slow with rough stones, then graduate to very fine (280) to finish. Keep it cool. We usually work to get the taper square and quit. Let it cool. Measure for coating.

We bend the oil squirts, but that's not for everybody. You have to break a few to know when and where to stop. Modified hone shoes get behind the squirts.

And we always torque plate... y'know, we don't make this easy, do we?
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1007 Olive St.
Lockwood, MO 65682
417-232-4634
FranksTDIs@sbcglobal.net

'02 80k grey leather, 99.5 R.I.P 153k
'85A2 NA 375k, '91 A3 290k Always Silver, Always a Jetta
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