A piston ring is a split ring that fits into a groove on the outer diameter of a piston in a reciprocating engine such as an internal combustion engine or steam engine. The three main functions of piston rings in reciprocating engines are : Sealing the combustion/expansion chamber. Supporting heat transfer from the piston to the cylinder wall. Regulating engine oil consumption. The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore.