2. Bearing failure can be caused by lack of lubrication, dirt or other foreign particles, motor overload or corrosion (pic. 14.2). Whatever the cause of the bearing failure, it should be corrected before the motor is reassembled to prevent recurrence.
Pic. 14.2. Typical bearing failures
3. When inspecting the bearing shells, remove them from the cylinder block/crankcase and main bearing caps and from the connecting rods and bearing caps of the lower connecting rod heads. Lay on a clean surface in the same order as they are in the engine. This will give you the opportunity to determine which crankshaft journal corresponds to a particular defective bearing. Do not touch the inner surfaces of the bearing shells with your fingers when checking them, as you may scratch this very «tender» surface.
4. Dirt and other foreign matter enter the engine in various ways. They may have remained in the engine after assembly, or passed through filters or the crankcase ventilation system. They could get into the oil, and from there into the bearings. Metal chips are often present from machining and normal engine wear. Small abrasive and metal particles sometimes remain in the engine after repair, especially if the cleaning was performed poorly, using incorrect methods of cleaning. Regardless of the cause, these foreign objects often bite into the soft bearing material and are easily detected. Large particles will not embed into the bearing structure, but will scratch the bearing and journal to some extent. The best measure to prevent this cause of bearing failure is to thoroughly clean all parts and keep the engine assembling with impeccable cleanliness. Frequent and regular engine oil and filter changes are also recommended.
5. Lack of lubrication (or decomposition of the lubricant) has a number of related causes. Overheat (which thins the oil), overload (which squeezes oil from the bearing surface) and oil leaks (due to increased bearing clearances, oil pump wear or high crankshaft speed) - all these factors contribute to the decomposition of the lubricant. Plugged lubrication channels, which is usually the result of misaligned lubrication holes in bearing shells, is also a cause «oil starvation» bearings and lead to their failure. If the cause of bearing failure is lack of lubrication, the soft working material is rubbed off or extruded from the steel base of the bearing. The temperature can increase to the point; in which the steel base turns blue from overheating.
6. In addition, the bearing life can be influenced to a certain extent by driving style. Wide open throttle, low speed operation (low speed) imposes very high loads on the bearings, causing the oil film to be squeezed out. These loads cause the bearings to deform, resulting in fine cracks on the bearing surface (fatigue failure). Ultimately, the bearing material no longer adheres so strongly to the steel base and breaks away from it.
7. Short trips lead to bearing corrosion because the engine does not generate enough heat to remove condensed water and corrosive gases. These products collect in engine oil, forming acid and sludge. When oil is supplied to the bearings, the acid attacks the bearing material and corrodes it.
8. Incorrect installation of the bearing when assembling the motor also leads to a malfunction. Tightly fitted bushings give insufficient running clearance, leading to «oil starvation». Dirt or foreign particles that have collected behind the bearing shell cause localized «bulges», that lead to a malfunction.
9. Do not touch the inner surfaces of the bearing shells with your fingers when assembling them, as you may scratch this very «tender» surface or cause dirt particles to deposit on them.
10. As mentioned at the beginning of this paragraph, bearing shells should be replaced without fail during an overhaul. Usage «old» liners is a false economy.