Aircraft Systems: Reciprocating Engine Piston Rings Purpose of the piston rings is to prevent leakage of gas pressure from the combustion chamber and reduce to a minimum the seepage of oil into the combustion chamber. Aircraft engine piston rings are made of high-grade cast iron; they must fit the cylinder wall snugly enough to provide a gas tight fit. Chrome-plated mild steel piston rings are used in the top compression ring groove. Piston Rings - Aircraft Mechanic School Study Supplement The number of compression rings used on each piston is determined by the type of engine and its design, although most aircraft engines use two compression rings plus one or more oil control rings. The cross-section of the ring is either rectangular or wedge shaped with a tapered face.