A piston for use in a hydraulic or other fluid operated device having sealing means intended to prevent the passage of working fluid between the piston and cylinder in which it moves consisting of the combination of at least one O ring or other sealing device arranged around the circumference of the piston and left and right hand intersecting helical channels formed over the length of the surface of the piston wall between the sealing device and the end of the piston subject to pressure of the working fluid. The helical channels act both to retain lubricating fluid and to provide a partial seal against the passage of working fluid between the piston and the cylinder, substantially complete sealing being effected by the sealing device.