A hollow piston for a hydrostatic piston machine and especially for a swash-plate-type axial-piston pump or motor, comprises a hollow outer cylinder member which is formed at one end with a ball head and at the other end with a cover. Extending centrally through this hollow piston is a central post having its opposite ends secured to or engaging the opposite ends of the outer piston member and formed with a central bore communicating between a passage in the cover and a passage in the ball head. At least one of these ends of the central post is juxtaposed with a respective end of the piston and the juxtaposed ends are formed with a conical surface and an edge engaging this surface to seal the post to the outer member.