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How to… fit new big end and main bearings

Don't say we only help with the simple stuff

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

How to… fit new big end and main bearings

By Alan Dowds

OKAY, we had some moans about our how-tos being a bit easy. We all have to learn somewhere though – and not everyone can be a MotoGP-spec spanner god like you (yes, we mean you).

So, here's a big 'un for you. A vital part of a major four-stroke engine rebuild is fitting new shell bearings on the crankshaft journals. Here's how (and why) to do it…

1. Know your journals

Plain journal bearings are half-circles (well, sort of hollow cylinder halves) made of a special soft metal. They're used in most crankshafts in two places: the main bearings, which sit in the crankcases, and hold the crankshaft steady as it spins. The other place is at the 'big end' of the conrods which connect the pistons to the offset arms on the crankshaft. The places where the bearings face the crankshaft are called the journals – a four-cylinder engine (usually) has five main bearing journals and four big end journals.



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