Crankshaft Main and Connecting Rod Bearings Condition Inspection

Although the crankshaft main and connecting rod bearings must be replaced as a matter of course during an engine overhaul, the old bearing shells should be retained for careful examination of their condition, the results of which can provide much useful information about the general condition of the engine. The illustrations show examples of typical bearing shell defects.

Bearing failure may occur due to lack of lubrication, entry of dirt particles, engine overloading and corrosion. Regardless of the type of defect, the cause of bearing shell damage must be eliminated during the engine overhaul to prevent recurrence.

For inspection, remove the bearing shells from their seats in the engine block/connecting rod lower ends and main/connecting rod caps and lay them out in installation order on a clean work surface. Keeping the bearing shells organized allows the nature of the identified defects to be related to the condition of the corresponding journals.

Dirt and foreign particles can enter the engine in various ways. They may be left inside the block during assembly, or enter through the filters or the crankcase ventilation system. All particles that enter the engine oil eventually end up in the bearings. Metal particles generated during normal operation of internal engine components often become embedded in the soft bearing material. There is a high probability of abrasive traces in the bearings, especially when insufficient attention has been paid to cleaning the block after engine overhaul is completed. Regardless of how foreign particles enter the engine, they are highly likely to become embedded in the soft surface of the crankshaft bearing shells and are easily detected during visual inspection. Large particles usually do not remain embedded in the bearings, but leave visible marks in the form of scratches, pits and scoring on the surface of the bearings and the crankshaft journals. The best protection against this type of problem is careful cleaning of components after completing the engine overhaul and strict observance of cleanliness during assembly. Frequent, regular engine oil changes also significantly extend bearing service life.

Oil starvation may result from several different but often interrelated causes. For example, engine overheating causes the engine oil to thin and be forced out of the bearing working clearances. Insufficient bearing lubrication may be caused by excessive working clearances as well as normal leakage (internal or external). Forcing oil out of the bearing clearances is often caused by constantly exceeding the engine speed limit. Blockage of the oil passages (usually caused by incorrect alignment of holes during component installation) also reduces the lubricant supply to the bearings. A typical result of oil starvation is complete or local wear/spalling of the bearing surface layer from the metal backing. In this case the operating temperature may rise to such a level that the backing takes on a bluish discoloration due to overheating.

The driving style of the vehicle owner also has a significant effect on bearing service life. Driving at low speed in a high gear places heavy loads on the bearings, which forces the oil film out of their working clearances. Such overloading increases the plasticity of the bearing shells and causes cracks in the surface layer (fatigue deformation). The surface material then begins to crumble and separate from the steel backing. Operating the vehicle in city conditions (frequent short trips) leads to bearing corrosion, because insufficient engine warm-up causes condensation and the release of chemically aggressive gases. These products accumulate in the engine oil, forming sludge and acids. When such oil reaches the bearings, the aggressive substances promote corrosion of the bearing shells.

Incorrect installation of bearing shells during engine assembly can also cause their rapid failure. An excessively tight fit does not provide the required bearing working clearance, which leads to oil starvation. Foreign particles trapped under the bearing shells during installation cause raised areas, and the surface layer around them wears away quickly.