A - Scratches due to dirt: contamination is embedded in the bearing material
B - Lack of oil: the contact area is worn out
C - Incorrect installation: light polished areas
D - Neck taper: the contact area is abraded by the neck itself
E - Lateral wear
F - Fatigue failure: pitting or scuffing
Bearing failure occurs due to lack of lubrication, the presence of dirt and other foreign particles, motor overload and corrosion. Regardless of the cause of bearing failure, before assembling the engine, the cause must be eliminated so that it does not happen again.
For inspection, remove the bearings from the cylinder block and from the covers, remove from the connecting rods and connecting rod caps, and then lay them out on a clean surface in the same order in which they are installed in the engine. This will match bearing problems to the corresponding crankshaft journal.
Dirt and other foreign particles enter the engine in many ways. They may remain in the engine during reassembly, or penetrate filters or the crankcase ventilation system (PCV). Dirt can get into the oil, and from it into the bearings. Metal particles are often present from machining and due to normal engine wear. Sometimes after grinding engine components, abrasive particles remain, especially if the parts are not thoroughly cleaned by appropriate methods. Whatever the source, these foreign objects often become embedded in the soft bearing material and can be easily located. Large particles do not penetrate the bearing, but scratch or damage the bearing and journal. The best way to prevent this cause of bearing failure is to thoroughly clean all parts and keep all components spotlessly clean when reassembling. Frequent and regular oil and filter changes are also recommended.
Lack of lubrication (or stop lubrication) have several related causes. Overheat (which reduces the thickness of the lubricant layer), overload (which squeezes oil out of the gap between the sliding surfaces) and oil leakage or leakage (due to increased bearing clearances, oil pump wear or high engine speeds) - all of these causes contribute to the lack of lubrication. Clogged lubrication channels, usually caused by misaligned lubrication holes in the bearing shell, also lead to insufficient lubrication of the bearing and bearing failure. If the cause of bearing failure is a lack of lubrication, the bearing material will abrade or peel off the steel backing. The temperature can reach a point where the steel substrate turns blue from overheating.
Driving style can have some effect on bearing life. Driving with wide open throttle at low speeds (overloading the engine) creates very large loads on the bearings, which tend to squeeze out the oil film. These loads bend the bearings, resulting in fine cracks on the surface of the bearings (metal fatigue failure). Sometimes the bearing material weakens in some places and breaks off the steel backing. Short trips lead to corrosion of the bearings, because when the engine is not warmed up enough, condensate and corrosive gases cannot be expelled. These products collect in the engine oil, forming acid and sludge. When oil enters the engine bearings, the acid attacks the bearing material, causing it to corrode.
Incorrect bearing installation during engine assembly also leads to bearing failure. Tight bearings leave little clearance for lubrication, resulting in insufficient lubrication. Dirt or foreign particles on the back surface of the bearing shell will cause raised areas on the bearing surface which will cause failure.