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 chamber so that there is minimal loss of gases to the crank case.
• Improving heat transfer from the piston to the cylinder wall.
• Regulating engine oil consumption by scraping oil from the cylinder walls back to the sump.