Piston rings have a tricky job to perform. We have to achieve a delicate balance between oversealing and undersealing the chamber. If we overseal the chamber, we risk engine damage via detonation and friction. if we underseal, we lose the important heat energy and pressure necessary to make power.
To achieve proper cylinder sealing, we have to understand piston rings, the materials they are made out of, and what happens to rings in operation. The top compression ring has the toughest job: to seal in combustion gasses and thermal energy. It also carries the heat of combustion to the cylinder wall and water jacket. The second compression ring assists the top ring in containing hot gasses. It also grabs some of the oil from the cylinder wall for lubrication purposes, even though it isn't called an oil ring.
The oil rings, as their name implies, carry oil up and down the cylinder wall. As the rod journal comes around, it throws oil on the cylinder wall. This is called splash lubrication. The oil rings wipe the cylinder wall with the splash lubrication.
Piston ring materials have evolved for improved durability. As a rule, compression rings have always been made of cast iron, ductile iron, or steel. What has changed is how these rings are treated in manufacturing for durability. Chrome piston-ring sets have a top compression ring that is chrome-plated for reduced wear and scuffing at the top of the bore. We see this a lot in racing applications.
Moly piston-ring sets sport a top compression ring impregnated with molybdenum, which has a very high melting point of nearly 5,000 degrees. This reduces the potential for scuffing. Because molybdenum is a lubricant, it helps piston-ring life immeasurably.
Checking compression ring...
Checking compression ring end gaps is important on each and every cylinder because no two bores or ring sets are the same. Ring end gap is there to allow for ring expansion from heat.
In the old days, piston-ring widths were typically 5/64-, 5/64-, and 3/16-inch. Racing engines went with lighter stock: 1/16-, 1/16-, and 3/16-inch ring packages. These days, it gets even thinner in the interest of friction and weight reduction.
Piston-ring-groove design is also important to smooth ring operation. We want ring grooves that offer stability without causing ring bind. Rings have to maintain specific clearances in the grooves to keep a smooth marriage with the cylinder walls.
Keeping proper cylinder sealing has always been a challenge for engineers. This has been approached with different kinds of materials and manufacturing techniques. One means to maintaining good piston-ring contact with the cylinder wall has been gas jets drilled into the piston crown and top-ring groove. When fuel ignites above the piston, heat and pressure enter the gas jets and force the top compression ring outward against the cylinder wall. This is intended to maintain cylinder pressure during initial light-off. This works well in racing applications, but doesn't live long on the street.