A hollow pin for connecting the piston and the small end of the connecting rod. With the rotation from the crank, the connecting rod that swings and the piston that moves linearly are connected without generating a moment. It is the part of the engine that has the highest surface pressure and is difficult to lubricate, but on the other hand, weight reduction is also extremely important. Therefore, special steel is used, and after quenching the surface, it is polished to maintain toughness and wear resistance. Generally, it is one-fourth to one-fifth the diameter of the piston.