During engine overhaul, it is important that the old main and big end bearing shells be retained so they can be examined closely because often they will give valuable information regarding the condition of the engine. Bearing failure may be caused by insufficient lubrication, dirt or foreign particles, engine overloading and corrosion so it should be corrected before re-assembly to avoid recurrence. When examining the bearing shells, they should be removed from their respective locations and laid out on a clean surface in the same orientation that will facilitate in matching any issues with the crankshaft journal. Foreign matter can also get into the engine during assembly, through filters, or as a result of machining operations and get embedded in the soft bearing material, or larger particles cause scoring. To avoid this type of failure, it is important to thoroughly clean all parts and change the oil and filter on a regular basis. Lack of lubrication can be caused by excessive heat, overloading, oil leakage, clogged oil passages, causing severe damage such as extrusion of bearing material from the shell. Driving habits, such as full-throttle low-speed operation, may also have a negative effect on bearing life due to fatigue failure, and short-distance driving may result in corrosion due to lack of heat. Wrong shell refitting during assembly can lead to oil starving and broken due to trapped dirt. It is important not to touch the bearing surfaces with fingers in order to avoid any scratches and contamination. For main bearings, there are three grades of shell thickness, which are indicated by colour coding, and should be chosen on the basis of the markings stamped on the cylinder block and crankshaft. Big end bearing shells are required in one size only and it is a good idea to fit new shells when performing an overhaul operation in order to ensure correct running clearances, provided that the crankshaft journals are in tolerance.