The piston skirt is made into an ellipse, which is called an inverse ellipse design. It is one of the methods to prevent skirt deformation. The main function of the piston skirt is to guide and bear lateral forces, so the design requirements of the piston skirt are: Ensure the matching clearance with the cylinder. In the early days, the piston skirt was made into a perfect circle, but when the engine was running, the piston skirt and the cylinder often had roughening in the direction of the piston pin. This was mainly caused by the relatively large deformation of the skirt in the direction of the pin axis during operation. When the piston is working in the engine, three deformations will occur in the direction of the skirt piston pin axis: lateral force. It is the surface of the skirt that bears the lateral forces, so the skirt has a tendency to be squashed, increasing its size in the direction of the piston pin. The explosive pressure and inertial force on the top of the piston. The combined effect of these two forces causes the piston crown to bend within the span of the piston pin, making the entire piston larger in size in the direction of the piston pin seat. Thermal expansion due to high temperatures. Among them, the piston pin seat part has a thicker wall and greater rigidity than other parts, so it deforms more seriously when thermal expansion occurs.