A piston 10 for an engine has a number of elongate pockets 21, 22, 23, 24 each of which is arranged at an angle relative to a longitudinal axis HP2 of a contact zone 15 on the thrust and anti-thrust sides of the piston 10. The pockets 21, 22, 23, 24 are arranged in two columns C1, C2 one on each longitudinal side of a central portion 20 of the contact zone 15 to form partial barriers to reduce the loss of oil from the central portion 20 during use of the piston 10. The pockets 21, 22, 23, 24 are oriented differently above and below a transverse axis HP1 of the contact zone 15 so as to reduce the loss of oil irrespective of the direction of motion of the piston 10.