Piston ring

A piston ring is a component that fits into a piston ring groove. Its role is to maintain airtightness of the combustion chamber on the compression and explosion strokes, scrape excess oil off the cylinder wall to prevent it from entering the combustion chamber, and transfer piston heat to the cylinder wall.
Piston rings are typically made by cutting parts from rings made of special cast iron, but steel rings have come into use recently to improve followability to cylinders.
Furthermore, outer surfaces of some piston rings are hard chrome plated or subject to nitriding treatments to improve wear resistance. Piston ring outer diameters are slightly larger than piston diameters to ensure they fit tightly to cylinder walls by their own elasticity of attachment to the pistons.
Piston rings include compression rings attached to the top of the groove and oil rings attached to the bottom of the groove.