Pistons are slightly smaller than the cylinder bore they fit into in the engine block. It allows for expansion as the engine warms, and the pistons fit into their bores. Piston rings fill this changing 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 contains three pieces; a spacer and two scraper rings that regulate a thin film of oil that lubricates the cylinder wall.
The rings fit into grooves machined into the pistons. The two top grooves are for the compression rings, and the bottom groove contains the oil control rings. Clean the pistons in a cold tank and clean these grooves with a ring groove cleaner before measuring. The thickness of this groove is essential. The compression ring grooves allow for just enough compression to force the ring both downward and outward. This force creates the tight seal needed to contain compression and prevent blowby. Use a new ring and a feeler gauge to obtain the groove's measurements. Place the feeler gauge between the ring and the top of the groove. Piston ring groove clearance is typically (.001 - .005).
Compression rings typically have a tapered or barrel shape. Because of these unique shapes, they must be inserted into the piston facing in the right direction. A dot is stamped into the top of the compression ring. It must face upward toward the dome or top of the piston. Check for the manufacturer's specifications before proceeding.
Piston rings are made of steel, 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 they're used in heavy-duty and off-road applications, where dirt may enter the intake and scuff the rings.