The main and connecting rod bearing shells should also be replaced during the engine overhaul when possible but if the old ones are kept it can be very informative on the condition of the engine. The stresses, which lead to bearing failure include lacks of lubricants, presence of dirt or any other foreign particle, engine overloading, and corrosion. This must be done irrespective of the reason before the unit is reassembled so that it does not recur. While comparing the bearing shells, take them out of their positions and place them on a flat surface so that any defects are aligned with the corresponding crankshaft journal while handling avoid touching the bearing surface in order not to introduce marks on it. Contamination may occur in the assembly line, by filters, and the crankcase ventilation system may contain machining metal chips, or abrasive material left during reconditioning. The former can become lodged in the soft bearing material while the latter scratch or gouge the shell and journal. Prevention is the key in this situation; that includes thorough cleaning of all parts and oil and filter changes. Common causes of lack of lubrication include overheating, overloading, leakage of oil, or obstruction of oil-feed passages so that the bearings heat up and wipe bearing material from the shell's backing. Driving also affects bearing life, acceleration, and deceleration at low speeds put a lot of pressure on the bearings thus causing the oil to be squeezed out; short trips also put a lot of stress to the bearings by not heating up at adequate levels. During assembly incorrect fitting leads to problems such as; the clearing between shells is reduced due to tight fitting shells and trapped dirt results to formation of high spots. One has to be very keen when reassembling bearing surfaces in order not to scratch or contaminate them.