What is the function of a piston ring?

• A. The seal between the piston and the cylinder wall (to reduce friction)
• B. The seal between the cylinder head and block
• C. The seal between piston and crankcase
• D. The seal around the piston pin

Answer: (A)

Piston rings are placed in grooves around the piston so they press against the cylinder wall as the piston moves. Their main job is to seal the combustion chamber, preventing high-pressure gases from leaking into the crankcase and keeping the oil film where it should be. This sealing also helps maintain piston cooling, since the rings conduct heat from the piston to the cooler cylinder wall. The phrase about reducing friction reflects a beneficial side effect: a proper oil film between the ring and bore lowers wear and friction, but the primary function is sealing and oil control. The other options describe seals that belong to the head gasket, the crankcase, or the piston pin area, which are not the function of piston rings.