A piston pump & motor is a type of reciprocating pump that moves and pressurizes fluid by using one or more reciprocating pistons; it is typically driven by an electric motor through a crankshaft and connecting rod. Though there are many types, all piston pumps feature at least one piston moving in an enclosed cylinder. 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. http://www.phpfluid.com/piston-pump/hydraulic-piston-pump/