How to… fit new big end and main bearings - 2

2. Under pressure

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Visordown's picture
Submitted by Visordown on Fri, 01/09/2017 - 14:29

The important part is the oil pressure inside the engine: this is piped to the crankshaft bearings, and it's this pressure that makes the plain bearings work. The crank actually spins on a cushion of high-pressure oil, so the bearings should never really wear in normal use – there's a little bit of contact when the engine starts, but this will quickly stop once the oil pump starts working.

If the oil pressure drops, or the oil is too old to keep its friction-busting powers, then the bearings start to touch the crank journal surface, and the bearing shell quickly wears away.

Our picture shows and old bearing shell on the left and a new one on the right. 

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