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How pistons work (part 2)


by Henri Lebarbé ,

Piston rings are vital as they seal the combustion chamber while withstanding extreme heat and friction caused by the movement of the piston inside the cylinder. Nowadays, motorcycle engines can reach alarming speeds, some in the neighborhood of 20,000 rpm. In racing, bikes have reached those engine speeds for decades. Granted, they're used over shorter periods of time. Lubricants are greatly responsible for this performance and it's no wonder: engine oils have evolved at an incredible pace in recent years.

Pistons in 4-stroke engines usually have three rings, while 2-stroke mills only have one, maybe two. In the former case, piston rings are separated based on their function. The top two, while also controlling oil, are primarily for compression sealing (compression rings); the lower ring is for controlling the supply of oil to the liner which lubricates the piston skirt and the compression rings (oil control rings). Typical compression ring designs will have an essentially rectangular cross section or a keystone cross section. The periphery will then have either a barrel profile (top compression rings) or a taper napier form (second compression rings).

Oil control rings can be one-piece or three-piece. In the latter case, the rings usually consist of two rails or segments (thin steel rings) with a spacer expander spring which keeps the two rails apart and provides the radial load.

Piston rings are subject to wear as they move up and down the cylinder bore. To minimize this, they are made of wear-resistant materials such as cast iron and steel, and coated or treated to enhance their wear resistance. Compression rings, in particular, need to remain lubricated even under extreme heat. Should there be a leak in the cooling system, a thermostat failure or a fan breakdown, engine overheating will quickly ensue. Riding in traffic on ultra-hot days is another cause for concern. Therefore, you have to perform an oil change within the recommended deadline, use the right viscosity and comply with the standards of the manufacturer.

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