U.M.R. Engines

MAIN BEARINGS - SEIZED / PICKED UP

When engine bearings seize it is the end result of major damage caused by a breakdown in lubrication or assembly bearing clearances being under minimum specifications.

The clearance is referred to as "vertical oil clearance". As the lubrication breaks down and allows metal to metal contact between the shaft and the soft bearing material. Heat created by friction melts and welds bearing material on to the shaft. In a very short period the bearing and shaft can be destroyed.

This same scenario takes place during a dry start after re-assembly. Engines have to be primed to obtain oil pressure before the initial start after a rebuild. This involves cranking the engine until oil pressure is obtained. Removal of spark plugs in a petrol or removal of injectors or glow plugs in a diesel allows oil pressure to be obtained at starter cranking speed.

The results of a dry start may not always destroy the bearings immediately but even minor bearing contact with the shaft will damage the bearing enough to result in premature failure. (note well, if an engine will not turn freely by hand or under normal starter motor action the engine should be dismantled until the source of incorrect clearances is identified and repaired. Common causes of a tight engine are incorrectly fitted wick rear mains, misaligned con rods, dirty assembly, overlapped oil rings, insufficient ring end or back clearance etc, etc.) If crankshaft-bearing seizure occurs many thousands of miles after initial engine assembly the failure is usually related to a breakdown in lubrication. (ENGINE DRIVEN WITH LOW OIL LEVEL OR NO OIL PRESSURE)

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