Abstract: A piston type fluid pump structure providing a selective variable operating fluid displacement, and including a piston having an operable travel, within a cooperable cylinder bore, through a pump stroke which may be of fixed length, a fluid supply port being provided which communicates with the interior of the cylinder bore when the piston is in a retracted position and a fluid outlet port communicating with interior of said bore through which fluid therein may be discharged during such pump stroke, an expansion chamber being provided in communication with the cylinder bore adjacent the discharge port therein, the effective volume of the expansion chamber being selectively variable whereby the output displacement of said pump structure may be varied by controlling the volume of said expansion chamber, and therewith the amount of fluid received therein during a pump stroke.