Most modern automotive engines have three rings located in grooves in the piston near the piston head. The top ring is the main compression ring, and forms a tight seal when combustion pressure acts on the ring, forcing it outward against the cylinder wall. Although considered a compression ring, the second ring is actually more important for oil control. When the engine runs oil is thrown off by the connecting rods onto the cylinder walls to lubricate the piston skirt and rings. However, only a minimal amount of oil is required for the rings, and the second ring scrapes off excess oil to prevent smoking, detonation and excess emissions that could be caused if the oil burned in the combustion chamber. The third ring is the main oil control ring. Most oil control rings are three piece rings, with upper and lower rails and an expander that maintains tension on the rails.

For optimal ring seal and long life, the cylinder wall finish must match the type of ring and ring facing that will be used. A plateau finish, where the peaks left by the previous honing step are removed, is best. Ideally, cylinders must be round, straight and have no taper. On performance engines, a torque plate is used when the cylinders are honed to simulate the block distortion created by torqueing the cylinder head. Compression rings are split so they can be installed in the piston ring grooves, and this leaves a gap when the piston and rings are installed in the cylinder. This gap must be within specification or excessive blow-by can result. Top compression ring gap should be 0.003-0.004” per bore inch on standard and moderate performance engines, with higher performance and racing engines requiring larger gaps.

Piston rings seal the floor of the combustion chamber between the pistons and cylinder walls, to ensure optimal benefit from combustion pressure and minimal power robbing blow-by. They also transfer heat from the pistons to the cylinder walls so it can be passed onto the cooling system, to keep the pistons within their specified operating temperature range. Finally, they prevent excess oil from entering the combustion chamber.

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Rings on modern engines are thinner and narrower to reduce friction. Ring material depends on application and the ring’s location on the piston. Since the top compression ring bears the brunt of the heat and pressure of combustion, most are made of ductile iron or steel. To resist wear, most top rings have a face coating of plasma molybdenum or chromium. Plasma molybdenum, usually called plasma moly, is porous, which allows it to retain oil for lubrication, and it enables rings to conform to the cylinder walls quickly, for faster break-in. Chrome-faced rings are very durable and ideal for engines operated in dusty conditions like off-road racing. Some OEMs are even giving top rings a gas-nitriding treatment that will allow the rings to last for up to 200,000 miles. Second rings are not exposed to as much heat as top rings, and since their main function is oil control, they can be made of cast iron.

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