It can be a hassle filing...
It can be a hassle filing three sets of rings per cylinder, but when done correctly the results can mean more power and less fall-off before the next rebuild.
Most of us consider piston rings little more than a necessary evil. That's probably because grinding rings is a time-consuming chore. And who hasn't accidently ground a gap too large and wasted both time and money ordering up a replacement?
But when done right, piston rings can create a power advantage by maximizing dynamic engine compression while still maintaining good oil control. This involves both proper selection for the intended use and proper preparation when gapping your rings. To get more information on making your piston rings work for you, we spoke with engine builder Keith Dorton, owner of Automotive Specialists.
The purpose of piston rings is to seal the gap between the sides of the piston and the cylinder wall. On almost all stock car pistons there are three rings. The top ring is in direct contact with the combustion flame and sees the most heat. Because of that, it usually has a moly coating to make it better able to withstand the harsh environment. For racing, the primary purpose of the top ring is to effectively seal the combustion chamber to maintain compression.
Here is a ring that hasn't...
Here is a ring that hasn't been ground properly. As you can see, the ends are no longer square to each other. It may not seem like much, but this can lead to a power-robbing loss of combustion pressure.
In OEM applications where a set of rings must be expected to last over 100,000 miles, the second ring is also expected to help maintain compression. But for racing, this isn't an issue. Instead, the second ring mostly helps with oil control. You do not want the second ring to provide too much of a seal because in a high-rpm application like racing, combustion pressure that passes the top ring and gets caught by the second ring can lead to ring flutter in the top ring. This should be avoided at nearly all costs.
The second ring is typically constructed from softer ductile iron because it doesn't see the heat that the top ring is subjected to, and also because the softer material helps the ring seal to the cylinder wall. The face of the ring also typically has a taper to it so that only the bottom portion of the ring actually touches the cylinder wall. This helps it function better as an oil scraper. The taper wears over time, and once it wears so much that the entire face of the ring is in contact with the cylinder wall, the ring's usefulness is long gone.
Although Dorton uses a powered...
Although Dorton uses a powered ring grinder that greatly speeds up the process, a manual crank-style grinder, such as this one from Powerhouse Products, works just as well (if you are willing to be patient) and costs a lot less.
Some high-end ring packs will also feature a second ring with a Napier-style design. A Napier ring has a "ridge" that steps up so that the inside of the ring is thicker than the outside. Dorton says that the effect is to provide more material inside the ring land for increased rigidity, while the face of the ring that contacts the cylinder wall is thinner to reduce sliding friction between the ring and the cylinder bore. Dorton says the Napier design works but, as you might expect, the extra engineering does come with extra costs attached. Usually, Dorton only uses Napier-style rings in dry sump applications where the vacuum created in the crankcase helps keep oil out of the combustion chambers.
Finally, the oil ring is actually an assembly of two scraper rings and a center expander ring. As it's named, the oil ring's only job is to pull oil off the cylinder walls and dump it into the oil pan.
Most ring packs for street applications come pre-gapped. But that simply isn't precise enough for racing where gaps should be accurate to within 0.001 inch or less. The purpose of the gap is to allow enough room so that the ring can expand once the engine is hot so that the ends almost touch. The trick is to get the gap as small as possible when the engine is hot without the ends butting. It's better to go too large rather than too small because if the ends do butt, it will lead to scuffing the cylinder walls, micro-welding of the ring to the piston's ring lands, and possibly even breaking the ring.