When tearing an engine over, it is required to replace both main and connecting rod bearings but it also is advantageous to keep the old bearings because they can tell a lot about the condition of the engine. Bearing failure develops from several causes, and they are as follows: poor lubrication, dirt, overloading of the engine, and corrosion, and before reassembling the engine, all these causes must be dealt with to prevent the repetition of the same. When inspecting the bearings they should be taken out from their rightful places and placed on a flat surface in a bid to match any defect with the crankshaft journal. Sources of contamination can be parts of the assembly, filters, or PCV system and can include metal chips, and abrasive from the machining or reconditioning of the engine. These foreign particulate can be lodged within the bearing while the larger ones can produce scoring or gouging. To avoid such failures, one must ensure that for all the parts are cleaned and that the oils and filters are replaced as required. Some of the common causes of lubrication problems include overheating, overloading, leakage of oil or restriction of the oil supply which causes a severe condition known as 'mapping' where the bearing metal is removed from its base. The driving habits also have the best influence on bearing longevity; for example, constant low-speed full power can generate high loads that expel oil, frequent short journeys produce corrosive effects due to inadequate heat. Further, imperfect bearing installation may cause oil depletion or have high-profen spots that have also been seen to bring about failure of bearings.