Do you know how many miles / kms the crank has done? They won't last forever, even if you renew the 3 main bearings (centre one with integral seal) with a crank rebuild, you also have the big end needle rollers to think about, and eventually the fatigue life of the con-rod. What you should avoid at all cost is splitting your cases, renewing just the Left & Right seals, then discovering a short while later a much bigger crank job is required.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30