This bearing shows normal bearing wear. Small, hairline cranks on bearing surface, with relatively few signs of foreign particles are normal signs of wear.

Worn areas located only on the top bearing in the connecting rods, or on the lower bearing in the block are a sign of fatigue. Excessive loads caused by detonation due to advanced ignition, poor quality fuel, and excessive compression may lead to this type of damage. Lugging the engine when pulling heavy loads can also contibute to bearing fatigue.

This type of bearing damage is caused by dirt. Inadequate maintance because of infrequent oil changes and lack of maintanence to the air filter contribute sand and dust paritcles into the oil.
Unclean conditions when assembling an engine during an overhaul will also result in bearing damage of this type.

This phot shows a bearing suffering from excessive friction and fusing signs caused by lack of oil.
Some of the causes of lubrication failure are a blocked oil filter cartridge, broken oil line, failure of the oil pump, or excessively low oil level in the working engine.

This bearing shows signs of erosion caused by noxious acids in the oil.
Some of the causes are extreme flow of gases caused by blowby, fuel pump leaking fuel into the crankcase, oil contamination by coolant, and excessive opertating temperatures.

The block straightness must be checked and align honed or bored to correct this problem.

Bearing failure which produces severe wear on both the upper and lower halves of the bearing are usually caused by excessive distortion of the crankshaft. This may be a result of unusually high loading conditions and excessive speed. A bent crankshaft will also produce this type of wear.