If the crankshaft has been reground, check for burrs around the oil holes. Remove any burrs found with a file or scraper and thoroughly clean the holes and channels from chips.
Use a micrometer to measure the diameter of the main and connecting rod journals of the crankshaft and compare the results with the technical data. Measure the necks at several points, both in diameter and in length, this will allow you to identify ovality and taper, if any.
Inspect the oil seal contact surfaces at each end of the crankshaft for wear and other damage. If the seal journal is badly worn, the crankshaft may need to be replaced.
Wear of the connecting rod journal is characterized by metallic knocking sounds that occur when the engine is running under load, at low speeds and with reduced oil pressure.
Wear of the crankshaft main journals is characterized by strong engine vibration and a metallic knocking sound, which intensifies with increasing engine speed and a decrease in oil pressure.
Even if the main and connecting rod bearings are subject to replacement during a major engine overhaul, they must be carefully inspected; their defects can be used to judge the technical condition of the engine.
Bearing failure can occur due to lack of lubrication, dirt and foreign particles, engine overload, or corrosion. Correct the cause of bearing failure before reassembling the engine.
To inspect the bearings, remove them and lay them out in the same order in which they were installed on the engine. This will allow you to identify the corresponding crankshaft journal and make troubleshooting easier.
Foreign particles can enter the engine in a variety of ways. Metallic particles appear in motor oil as a result of normal engine wear. Small particles, along with engine oil, can get into the bearings and easily penetrate its soft material. Larger particles entering the bearing can scratch it or the crankshaft journal. To prevent bearing failure due to this cause, it is necessary to thoroughly clean all internal surfaces of the engine and keep them clean during assembly. It is recommended to follow the required frequency of oil and filter changes.
Insufficient lubrication of the crankshaft journals can be caused by many reasons: high oil temperature, engine overload, oil leakage and other reasons.
Driving style also affects the longevity of bearings. When the throttle valve is fully open at low engine speeds, the load on the bearings increases and the oil film is squeezed out of the contact area. High loads lead to the appearance of cracks in the working part of the bearing, which in turn can cause the antifriction layer to separate from the steel base.
Short-distance driving causes bearing corrosion because the engine does not reach a stable operating temperature where water vapor and corrosive gases are eliminated. Vapors and gases condense in motor oil, forming acid and sediment. The acid, along with the motor oil, gets to the bearings and causes their corrosion.
Incorrect selection of bearings during engine assembly also causes their failure. Bearings installed with preload have a clearance that is insufficient to ensure normal lubrication of the rubbing surfaces.