the present invention comprises a fluid pump motor unit and a controller unit which controls the output of the pump / motor irrespective of the speed of the power source driving the pump / motor unit . the pump / motor unit comprises a cylindrical housing within which there is arranged at least one rectangular expansible cylinder assembly mounted on a crankshaft in a radial pattern , the cylindrical assembly being pivotally mounted within the housing for movement about trunnions , the rectangularly adapted piston assembly within said housing being journaled on a crankshaft which crankshaft with respect to the centerline of the cylindrical housing has a rotation parallel to , but spaced from , the centerline of the housing . the pivotal oscillation of each cylinder assembly alternately opens a port for the inflow of a fluid and subsequently closes that port while simultaneously opening another port for the outward flow of the fluid under compression as the piston moves toward the top of the chambers . novel means are provided to seal the side walls of the cylinder with the end plates of the housing and additional novel means are provided to seal the piston and cylinder during the movement of the piston therein . the invention also provides for intake and outward flow of the fluid within the housing through the end plates by means of conduits contained therein . the controller unit encompassed within the present invention comprises a shaft keyed to the crankshaft of the pump / motor unit , a pair of rotatable sleeves , one within the other , the latter keyed to the shaft keyed to the crankshaft , the sleeves being counter rotatable with reference to each other , each having a helical groove therein substantially disposed 180 degrees along the circumference of each sleeve relative to each other and intersecting each other , the two grooves thus providing a camming surface in which a pin inserted within the two grooves can be moved by another pin operating in an arcuately inclined groove in another sleeve on a control rod which is rotatable between positions which place the crankshaft rotatable between positions which place the crankshaft of the pump / motor unit in aspects ranging from full forward position to neutral .