The Link is composed of link body, link big head cover, link small head bushing, link big head bearing bush and link bolt (or screw). The link group is subjected to the gas force from the piston pin, its own oscillation and the reciprocating inertia force of the piston group. The magnitude and direction of these forces are changed periodically. Therefore, the link is subjected to alternating loads such as compression and tension. The link must have sufficient fatigue strength and structural stiffness. Insufficient fatigue strength, often cause link body or link bolt fracture, and then produce the whole machine damage major accident. If the stiffness is insufficient, it will cause the bending deformation of the rod body and the circular deformation of the big head of the link, leading to the partial grinding of the piston, cylinder, bearing and crank pin.