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.