a radial piston pump is provided with a plurality of valve / piston cartridges . each cartridge includes the piston and at least one of the valves connecting the piston to the inlet and outlet passages of the pump . the bearings for the main rotor are pre - loaded using a threadedly adjustable end guill . once adjusted , the end quill is held in location by a pin or the like . the rotor itself is provided with counterweights to balance the eccentric cam surface . these features allow construction of a pump housing which is substantially a single unit , rather than two pump halves . a significant reduction in the number of machined bores also is obtained .