If the crankshaft has been ground, check for burrs around the oil holes. Remove any burrs found with a needle file or scraper and carefully clean the holes and channels from chips.
Using a micrometer, measure the diameter of the crankshaft main and connecting rod journals and compare the results with the technical data. Necks are measured at several points, both in diameter and in length, this will reveal ovality and taper, if any.
Check the oil seal contact surfaces at each end of the crankshaft for wear or 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 knocks that occur when the engine is running under load, at low speeds and a decrease in oil pressure.
Wear of the crankshaft journals is characterized by strong engine vibration and a metallic sound that increases with increasing engine speed, as well as a decrease in oil pressure.
Even if the main and connecting rod bearings are to be replaced during the overhaul of the engine, they must be carefully examined, their defects can be used to judge the technical condition of the engine.
The bearing can fail due to lack of lubrication, dirt and foreign particles, motor overload or corrosion. Correct the cause of the bearing failure before the motor is reassembled.
To inspect the bearings, remove and arrange them in the same order in which they were installed on the engine. This will identify the appropriate crankshaft journal and facilitate troubleshooting.
Foreign particles can enter the engine in various ways. Metal particles appear in engine oil as a result of normal engine wear. Small particles along with engine oil can get into the bearings and easily penetrate into its soft material. Larger particles entering the bearing can scratch it or the crankshaft journal. To prevent bearing failure from this cause, all internal surfaces of the motor must be thoroughly cleaned and kept clean during reassembly. It is recommended to observe the required interval for changing the oil with filter.
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 has an impact on bearing life. When the throttle is fully open at a low engine speed, the load on the bearings increases and the oil film is squeezed out of the contact zone. High loads lead to the appearance of cracks in the working part of the bearing, which in turn can cause the anti-friction layer to separate from the steel base.
Short distance driving results in corrosion of the bearings as a result of the motor not reaching a stabilized operating temperature that removes water vapor and corrosive gases. Vapors and gases, condensing in engine oil, form acid and sediment. The acid, along with the engine oil, gets into the bearings and causes them to corrode.
Incorrect selection of bearings during engine assembly also causes their failure. Bearings installed with a preload have a gap that is insufficient to ensure normal lubrication of the rubbing surfaces.