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. http://www.phpfluid.com/hydraulic-piston-parts/