The structure and shape of the piston pin is very simple, basically a thick-walled hollow cylinder. Its inner hole has cylindrical shape, two-section frustum shape and combined shape. The cylindrical hole is easy to process, but the piston pin has a large mass. The mass of the piston pin with two truncated cone holes is small, because the bending moment of the piston pin is the largest in its middle part, which is close to the beam with equal strength, but the machining of the cone holes is difficult.
The offset of the piston pin is to reduce the "slapping" of the piston on the cylinder when the piston changes direction at the top dead center. When the piston approaches the top dead center, when the gas pressure acting on the right shaft of the piston pin seat is greater than that on the left shaft, the piston leans outward, and the lower end of the skirt first changes direction and contacts with the cylinder wall, and then the head contacts with the cylinder wall, thus realizing two-step commutation. Two-step commutation can reduce cylinder knock. In this way, the piston can reverse before the top dead center, especially at the end of the compression stroke, and the reverse torque of the piston can "explode" in advance, which improves the working smoothness of the engine.