PISTON

PURPOSE:To reduce the hitting sound of a piston while preventing a seizure of the piston by making the outer formation of a piston skirt in a form to contract the diameter toward the lower end edge, and providing a metallic strut with a smaller thermal expansion coefficient than the piston main body and with a high rigidity, integrally at the part near the lower end edge. CONSTITUTION:At a high thickness part formed near the lower end edge 7 of a skirt 6, a material with a high rigidity and a low thermal expansion coefficient lower than the light alloy of the piston material, a strut 10 made of a steel, for example, is fixed integrally by the method of a casting, for example, and the upper part (a) of the skirt is contracted in the diameter gradually as coming to the upper side, making the clearance 17 between a cylinder wall 16 a little larger, and at the center (b) which is the largest diameter part of the strut 6, the clearance 17 is made to have almost a constant value from the upper side to the lower side, a little narrower than the conventional one, to make the outer diameter almost constant, while the lower part (c) of the skirt is contracted in the diameter relatively sharply toward the lower end edge, making the clearance 17 with the cylinder wall 16 larger.