Piston Ring
A split ring that fits into a groove on the outer diameter of a piston, sealing the combustion chamber and regulating oil consumption.
Piston rings are metal rings that fit into machined slots on the outer diameter of an engine piston. They serve three critical functions in an internal combustion engine: sealing the combustion chamber so that combustion gases do not leak into the crankcase (known as blow-by), transferring heat from the hot piston to the cylinder wall where coolant circulates, and scraping excess oil off the cylinder walls to prevent it from burning in the combustion chamber.
Most automotive pistons utilize a set of three rings. The top ring is the compression ring, which seals the combustion pressure. The middle ring is the scraper ring (or second compression ring), which assists in combustion sealing and scraping oil. The bottom ring is the oil control ring, which distributes oil evenly on the cylinder wall and scrapes back the excess into the oil pan.
Over time, piston rings can wear out, crack, or become stuck in their grooves due to carbon deposits from dirty engine oil. Worn or damaged rings allow oil to enter the combustion chamber and burn, producing blue-grey exhaust smoke. They also allow pressurized combustion gases to blow past the piston into the crankcase, causing low engine compression, power loss, and crankcase over-pressurization.
Diagnosing bad piston rings typically involves a cylinder compression test followed by a wet compression test (adding a small amount of oil to the cylinder) or a cylinder leak-down test. Replacing piston rings requires a complete engine disassembly, cylinder honing, and piston removal, making it a major, high-cost engine rebuild procedure.