A piston for a hydrostatic machine comprises a cam follower ball in a seating in a piston body. An aperture through the piston body provides access for working fluid to reach the seating to give lubrication and hydrostatic support. A low friction insert of synthetic plastics material in the seating provides a bearing surface for the ball. The insert is retained in the piston body by a lip which prevents or inhibits extrusion of the plastics material under working load pressure.