Abstract: A vane pump in which the clearance between the side plates and the rotor and the cam ring is maintained as a substantially constant dimension by hydrostatic means regardless of dimensional changes of the pump parts due to thermal conditions when the pump is operating. At least one of the side plates is flexible so that it can flex to maintain its proper clearance with both the cam ring and rotor despite variations in the thickness of the cam ring and rotor that may occur during manufacture of these parts and/or due to temperature differentials within the pump during operation.