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Spun a rod bearing?

 So you heard a knocking sound in the bottom end. You shut the engine off and tore it down. Your rod looks something like this and you are wondering "what happened?"...It may have even broke, but the big end is discolored like in the picture.The discoloration is a result of excessive heat. Almost without exception, this is caused by a spun rod bearing. Rod bearings spin due to a colapse of the oil film between the crank journal and rod bearing. Why this happens is a topic of great discussion. Mahle/Clevite has a very nice guide on their website to help you diagnose bearing wear patterns and failures. To view their guide, click HERE . When discoloration is visible (even a very small amount) the rod has been heated enough to compromise or ruin the heat treatment in the steel. This can have unpredictable effects on the strength of the rod. It is impossible to be sure if the rod can be used reliably once this has happened. The best course of action is to replace the rod. See our FAQ topic on single rods for information on ordering single rods.

 
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