Abstract: A piston comprises a ring land extending to a crown, a pin boss formed in a lower side of the ring land, a piston pin bore formed in the pin boss and a skirt section extending to the ring land. The skirt section is formed in such a manner that an area of a thrust side skirt is greater than an area of a counter-thrust side skirt. The thrust side skirt is formed so as to become thinner in wall thickness in a portion closer to the ring land and a portion farther from the ring land than in a central portion on both sides of a middle position along circumferential direction around a piston center line substantially perpendicular to an axis line of the piston pin bore and a wall thickness of a portion between the closer portion and the central portion and a wall thickness of a portion between the central portion and the farther portion vary gradually.

Abstract: In a piston, a circumferential groove for a piston ring, a knock pin being inserted in a bore set radially relative to the piston ring groove. A head end of the knock pin protrudes into a space defined by the piston ring groove for interposing between ends of a piston ring for preventing turning, or rotation, thereof. Means for preventing mechanical interference between the walls of the piston ring groove and the head end of the knock pin is provided.

Abstract: A piston structure includes protection of the peripheral edge portion of a lubricant injection nozzle receptacle cut-out. The piston structure employs mechanisms for reinforcing the peripheral edge portion of the cut-out to an extent that can avoid concentration of the stress and thus can avoid the formation of cracks.