A - Scratched by foreign particles - grains are visible, embedded in the working layer of the liner; B - Lack of oil - top layer is worn off; C - The inserts are incorrectly positioned during installation - there are shiny spots (polished) areas; D - The neck is tapered - the top layer is removed from the entire surface; E - Wear of the liner edge; F - Fatigue failure - craters or pockets formed
1. Although the main and connecting rod bearing shells should be replaced during an engine overhaul, retain the old shells for detailed inspection as they can provide valuable information regarding the condition of the engine. The shell size is stamped on the back and this information should be given to the supplier of new shells.
2. Damage to the bearings may be caused by insufficient lubrication, foreign particles getting on them, engine overload or corrosion (refer to the illustration above). Regardless of the cause, it must be corrected before reassembling the engine.
3. To inspect the bearing shells, remove them from the cylinder block/caps/connecting rods. Lay them out on a clean work surface in the same order as they are in the engine. This will help you determine which bearing is causing the problem.
4. Dirt and foreign particles enter the engine in various ways. They can be left in the engine after overhaul or pass through filters or the crankcase ventilation system. Often there are metal grains left after regrinding or formed during normal engine wear. Foreign particles that get into the bearings actively embed themselves in the soft working layer of the liners, where they are easy to detect. Large particles do not sink into the material of the liners, but scratch the liners and journals. The best way to prevent bearing failure for this reason is to thoroughly clean all components and maintain impeccable cleanliness during engine assembly. In addition, it is recommended to change the engine oil and filter frequently and regularly.
5. Lack of lubrication has many interrelated causes. These may include: overheating (makes the oil less viscous, which causes its layer on the bearings to become thinner), overload (squeezes oil from the bearing surface) or leaks (bearing clearances are excessive, oil pump is worn out or engine speed is high). Blockage of the lubrication passages, which is usually the result of misalignment of the bearing lubrication holes, also leads to a lack of lubrication and causes bearing failure. When a bearing fails due to lack of lubrication, the bearing layer is worn away or peeled off from its steel backing. Temperatures can rise to the point where the backing steel turns blue from overheating.
6. Driving style also affects bearing life. Full throttle and long hours at low revs (engine overload) place very high loads on the bearings, which causes the oil film to be pushed away from the liners. As a result, the liners bend and develop a network of small cracks (a "fatigue" failure). Eventually, the working layer of the bearing shell will break into pieces and break away from the steel base. Short trips lead to corrosion of the liners, since the engine does not heat up enough to repel compressed water vapor and corrosive gases. The vapors are adsorbed by the engine oil, forming acid and sludge. When this oil gets into the engine bearings, the working layer of the bearing shells oxidizes.
7. Incorrect selection of the bearing shell will also cause bearing failure. Too large shells that fit tightly into the bearing leave insufficient operating clearance, which leads to a lack of lubrication.
Text provided by an online resource: www.RenaultBook.ru
8. If new bearings are to be installed, the running clearances must be measured before the engine is assembled to ensure that the bearings are selected correctly (refer to SectionInspection of the crankshaft). If there is any doubt as to which bearing shells should be used, consult your Renault representative.