It may be due to the excessive clearance between the connecting rod bearing and the connecting rod journal, which leads to reduced oil film stiffness, reduced load-bearing capacity, deteriorated lubrication conditions, and boundary friction or dry friction, causing abnormal noise.

It may also be that the crankshaft main bearing is usually integrated with the shaft, while the lower bearing cover is manufactured separately, and then connected to the upper half by connecting rod bolts. The inner surfaces of the upper and lower halves of the bearing are equipped with bearings.

In the bearing structure, oil holes are drilled on the inner surface of the sleeve, and some oil grooves are designed to facilitate lubrication of lubricating oil. In addition, self-adjusting or platform-type connecting rod small head bearings are often used in the structure of Bare Engine bearings.

The function of Bare Engine bearings is to reduce the wear of the crank pin. By connecting the crank pin and the shaft, the friction between the crank pin and the shaft can be effectively reduced, thereby extending the service life of the crank pin.