The equations of motion of a conventional piston in axial piston pumps and motors with metallic contacts are derived. For swash plate types, the fluid film shape between the piston and the cylinder wall, contact forces and loss of power are numerically obtained in relation to both the operating conditions and piston dimensions. The length of the sealing part composed of the piston and cylinder wall is, in particular, shown to considerably affect the loss of power. Finally, the validity of the analysis is discussed by comparison with the results of a performance test of a pump for high water content fluids.