Patent ID: 8353267

Claim:
A piston for an engine having a stable behavior of a piston ring, the piston comprising: a body positionable in a cylinder; an upper ring, a middle ring, and an oil ring, which are provided at an outer circumferential portion of the body; and a ring-shaped groove in the body formed between the upper ring and the middle ring, wherein the ring-shaped groove has a deep portion which has a concave shape so that oil staying in the ring-shaped groove is uniformly smoothly flowed, and further comprising a first gap, having a shape of a straight line, formed between substantially parallel portions of a liner of the cylinder and an area of the body extending from the ring-shaped groove to the middle ring, and wherein the first gap is larger than a gap between the liner of the cylinder and an area of the body extending from a portion of the body adjacent the upper ring to the ring-shaped groove, and further comprising a second gap, having a shape of a straight line, formed between substantially parallel portions of the liner of the cylinder and an area of the body extending from the middle ring to the oil ring, and wherein the second gap is larger than the gap between the liner of the cylinder and the area extending from the portion of the body adjacent the upper ring to the ring-shaped groove.