The paper presents the results of an experimental and theoretical investigation of the effect of clamping ratio and orifice size on the performance of slippers in axial piston pumps and motors. Polishing of the running face to a slightly convex form appears to be essential for successful operation under all conditions. It is shown that slippers operate perfectly satisfactorily with the orifice blanked and, indeed, are generally most stable in this condition. Introduction of the orifice in under-clamped slippers increases the clearance. However it can also destabilise the slipper resulting in the slipper becoming sensitive to the effect of tilting couples. The effect on overclamped designs is more complex depending on the precise value of clamping ratio and on the width of the slipper land.