A piston such as is used in internal combustion engines, steam engines, gas engines, air engines, pumps, compressors and the like from which the usual piston rings are absent the sealing of the piston being affected by means of right and left hand intersecting helical channels formed on a portion of the surface of the piston wall.It is preferred that the helical channels terminate at each end in a circumferential groove. The sealing is effected by means of oil trapped by the channels between the piston wall and a cylinder in which the piston operates.