Piston Rings
Pistons are smaller than the bore they fit into in the engine block. This is because as the engine heats the pistons expand in their bores. Piston rings are used to fill this gap and contain the compression and expansion gases in the cylinder. Most automotive piston ring sets contain two compression rings and one oil control ring. The oil control ring will likely contain three pieces. A spacer and two scraper rings that control and allow the right amount of oil to lubricate the cylinder walls.
The rings fit into grooves machined into the pistons. The two top grooves are dedicated to the compression rings and the bottom groove dedicated to the oil control rings. Clean the pistons in a cold tank and clean these grooves with a ring groove cleaner before measuring. Use a new ring and a feeler gauge to obtain the measurements. The thickness of this groove is important. The compression rings located in the top two lands of many pistons allow just enough compression to force the ring both downward and outward creating the tight seal needed to contain the compression and prevent excessive blowby.
Compression rings are usually designed with a taper or a barrel shape. Because of these unique shapes they must be placed into the piston facing in the right direction. These are typically designated with a dot on the top of the compression ring that must face upward toward the dome or top of the piston. Check for manufacturers specifications. They are created of steel often coated with molybdenum (Moly Rings) or chrome. Cast iron rings are often used when rebuilding an older engine and chrome rings are used for their durability. Chrome rings are known to wear the cylinder walls so are usually used in heavy duty and off road applications where dirt may enter the intake and scuff the rings.
Moly rings are great because they last and are often installed OEM on new cars and trucks. Most technicians will be replacing the old rings with an OEM or original manufacturers ring set. To install most rings first install oil ring, followed by the second compression ring and then the top compression ring, and use a piston ring installer to prevent any damage to the new ring.
