Piston Ring

A piston usually has three rings. The top compression ring seals the combustion pressure, the second ring helps sealing and scrapes oil, and the bottom oil-control ring removes excess oil from the cylinder wall so it is not burned. Rings are made of cast iron or steel with special coatings and press outward against the cylinder wall. A small gap at the ring ends allows for expansion as the engine heats up.

Combustion pressure pushes the compression rings against the cylinder wall and the bottom of their grooves, sealing the chamber. The oil ring has slots that let scraped oil drain back into the sump through holes in the piston.

Worn or stuck rings let combustion gas into the crankcase (blow-by) and oil into the combustion chamber. Typical signs are high oil consumption, blue smoke under load, low compression and oily spark plugs.