Back in the good old days, piston rings were almost always cast iron. But cast iron has a few drawbacks–namely, it’s brittle–and over time steel rings were developed.
At first, steel rings were reserved for only big buck engine builds for professional race teams and the like. But steel rings have consistently dropped in price until they are actually quite affordable today. Steel rings are stronger and have better fatigue resistance than cast iron, and they work really well in the super-thin sizes that are becoming extremely popular.
But steel doesn’t behave well in a cast iron cylinder bore–which is the material for almost all engines. So steel piston rings require a coating of some type. At the AETC conference Bill McKnnight of Mahle Clevite gave us a rundown of the various available coatings available today to work with steel rings. Mahle doesn’t necessarily recommend every coating, but we tried to include all the potential coatings to help you sort the better from the best.
Right now the current best option is usually SAE 9254 steel, which is 15 percent stronger than ductile iron. It is also five percent stronger than stainless steel and 20 percent harder than stainless.
Chrome Plating — Made popular back in WW2 as a way to make cast iron rings live longer in harsh environments in Africa and other places. Today it has been surpassed by other coatings and is mostly only used on oil rings.
Moly — Standard moly is sprayed onto the ring from a piece of wire. It has very good heat resistance as well as oil retention. But it is also prone to flaking off, and if there is any abrasive material that gets into the bore (sand, metal, etc.) it can scrape off in very little time.
Plasma Moly — This is moly that is applied by plasma arc welding from a powder source. The powder can be blended with other materials to create a better bond strength as well as improved wear and heat resistance.
Better Plasma Moly — This is generation two of the plasma moly application process. In this case, ceramics are combined with the moly to create an improved bond to the ring and make for even more wear resistance.
HVOF — This stands for “High Velocity Oxygen Fueled” process. Moly’s trade name for this is HV385. It requires a big investment from the ring manufacturer but creates a coating with excellent bond strength as well as hardness and wear resistance.
GNS — A coating with a stainless base. It is extremely hard, which can be both good and bad, so you have to be careful.
PVD — This stands for “Particle Vapor Deposition.” This coating works with both stainless steel and SAE9254 rings. A very popular coating along with HVOF. It is hard like GNS but has better scuff resistance than GNS.