Moly or regular piston rings?

moly versus cast iron

Let me qualify myself a bit here before I get to replying to this.

I have been building street performance and pro-gas and similiar (drag engines) for over 25 years. The drag engines have always been in the winners circle and extremely durable.

So, here is my take on it for whatever it is worth. It's all about sealing, friction, and longevity. That's why I would not touch a cast iron (ductile iron) ring with a ten foot pole. It's only saving grace is extremely quick seating (sealing) properties after initial run in.

The final cylinder finish has to be courser for cast iron than it does for moly-based rings, that equates into two rough surfaces grinding each other into as perfect a fit as you can get with the engine running. Where does this very fine grit go? Early moly rings had a bad reputation for not sealing quickly, but as development and manufacturing tecniques became more sophisticated this was finally overcome. Cast iron wears the cylinder walls much faster than moly rings as moly has some self lubricating properties due to its composition.

Modern moly rings have been pre-lapped to some extent for fast sealing. The only thing that will prevent them from working and sealing immediately is the finish of the cylinder wall by whomever is doing the cylinder honing. This is a critical operation, not a slap it in, run a few hone passes, and call it good. Automatic honing machines are also a nemesis to moly unless the shop is consientous (spelling) about maintaining the machine and stone dressing and/or replacing the stones when it is needed. Many shops won't do this because it adds to their operating costs. This results in a bad finish, taper in the cylinder, and out of roundness. No ring likes to see this no matter what it is made out of.

Detonation, for whatever the reason, is extremely hard on the first two rings. Moly rings, because of their chemical composition, resist detonation degradation much better than plain ductile iron rings. Moly does not wear the cylinder as much. Moly is naturally self-lubricating, reduces friction, adds to the horsepower gainsof the engine, and exceeds any benefits that cast iron rings might have if properly installed.

Where has moly derived a bad name? From improper cylinder wall preparation. The rough honing stones cut the cylinder into it's near final dimensions, the next finer stone in the hone cuts the peaks down of the first stone without increasing the root diameter. Then comes finishing. Run the finishing (fine grit) honing stone which further flattens the peaks without increasing the root depth. The final passes of the honing operation are with the fine grit stones run with slighty reduced pressure to "plateau" the surface. If this is done correctly, moly rings will seal completely within the first 50 miles of driving, give you less friction and heat, and last far longer than cast iron.

If you want to prove all this to yourself, take some acetone tape and dissolve it in acetone. Paint this on the cylinder wall and let it dry. When it is dry, carefully peel it off and look at it under a microscope. You will immediately see why proper honing techniques are so important to sealing and longevity.

Now this last statement is probably going to set a bunch of you off, but I am going to say it anyway. Someone whom has developed a feel for hand honing, has presicion measuring instruments to measure the progress, can do a much better job off of cylinder honing than any automated cylinder hone in existance. If you doubt this, then why are the most accurate lathes/mills all hand scraped? Because the human fingernail can easily detect 1/2 thousands when scraped across a surface. With experience, this can easily be halved.

Get it straight, get it round, and moly reigns supreme without the wear associated with cast or stainless. The previous suggestion for using acetone based tape for "seeing" your honing quality came my working with Sealed Power many years ago.

The only thing that might be better than moly at this point in time is the ongoing development to perfect ceramic rings. Till then, moly and proper honing is the only way to go.

Dennis
De'Briga LLC

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Post by 10 Wheeler Rob » Fri Mar 01, 9:34 am

Link to Dongya

Teflon rings are probably the best choice for minimizing rust issues, they usually require a two or three peice piston design, since they will not expand and contract like an o-ring or metal ring for instalation. There are commecially made teflon rings made with the wiper shape, some with a spring metal part inside that make up of tolerances, creep and wear, these are wonderful seals for steam applications with. There some Teflon(TM) ring designs wich incorporate an o-ring also.

O-rings will work with the right bore and surface & gland tolerances and the correct surface finish, but they will have to be replaced perodically due to wear. Viton(TM) is proably the best choice for material. Silicone is not recomended for dynamic seals. If cylinders and or pistons are cast iron or steel then rust may be an issue, and could tear up o-rings very fast.

Bill Shields who contributes on here a lot has Teflon seals in Camel Back, I am sure he would be willing to share his piston & ring design.

All in all non-metalic rings work very well when tolerances and surface finishes are done right. They will not wear in like a cast iron ring will, all in all the old cast iron ring design is tolerant of larger surface finish and tolerance limits.

There is an Allen Mogal at our club with Teflon rings that has been running trouble free for several years. The owner just gives then a good squirt oil through the snifter valve ports running. I am not shure how the rings and pistons were configured.

Rob

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