Hydraulic piston parts are disc or short cylinder fittings closely within an engine cylinder in which they move up and down against a liquid or gas, used in an internal combustion engine to derive motion, or in a pump to impart motion. A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders, and pneumatic cylinders, among other similar mechanisms. It is the moving component that is enclosed in a cylinder and made gas-tight by piston rings. In an engine, its purpose is to transfer force from the expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod.

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