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In general, a piston is a sliding plug that fits closely inside the bore of a cylinder .

Its purpose is to change the volume enclosed by the cylinder creating compression of the combustible charge of air and fuel and when ignited at the proper moment, to transmit the power created during combustion to the crankshaft.

Most pistons fitted in a cylinder have piston rings. Usually there are two spring-compression rings that act as a seal between the piston and the cylinder wall - though the springiness of the ring does not in fact create the seal; the seal results from the downward pressure of the exhaust gas following ignition. The head of the piston can be flat, bulged or otherwise shaped. Pistons can be forged or cast. The shape of the piston is normally rounded. The piston is an important component of a moped engine. It is fitted to a connecting rod through a wrist pin.