Connect the piston and the crankshaft, and transmit the force on the piston to the crankshaft, converting the reciprocating motion of the piston into the rotational motion of the crankshaft.

The connecting rod group is composed of connecting rod body, connecting rod big end cap, connecting rod small end bushing, connecting rod big end bearing bush and connecting rod bolts (or screws). The connecting rod group is subjected to the gas force from the piston pin, its own swing and the reciprocating inertial force of the piston group. The magnitude and direction of these forces change periodically. Therefore, the connecting rod is subjected to alternating loads such as compression and tension. The connecting rod must have sufficient fatigue strength and structural rigidity. Insufficient fatigue strength will often cause the connecting rod body or connecting rod bolt to break, resulting in a major accident of damage to the whole machine. If the stiffness is insufficient, it will cause bending deformation of the rod body and out-of-round deformation of the big end of the connecting rod, resulting in eccentric wear of the piston, cylinder, bearing and crank pin.