ENOUGH OF YOUR LIP

Having bought his 500cc BSA B33 engine in bits, Steve Cantrell can’t work out from the manual which way the lipped roller main bearings fit. The outer races have a lip on one side of the bearing and there’s one in each crankcase half, with an additional ballrace outboard on the drive (left) side. The idea is that the lips are on the outside of both bearings. The bearing inners are an interference fit on the shafts and the outers in the crankcase, while the shaft is a sliding fit in the outer ballrace.

The lips exist to regulate lateral movement of the crank – they won’t tolerate much actual thrust, but in normal circumstances there shouldn’t be any because the crank is positively located at the ballrace bearing, by being tightened against the crankshaft shock absorber assembly. The open, unlipped sides of both outer races should face inward, so that when the cases are separated the crank can come out with its inner races intact. It’s not usually a problem, but there should be about three thou free endfloat on the crank before it’s tightened to the shock absorber. The ballrace is usually a tight enough fit to make this hard to check, but in any case make sure the crank spins freely without any tight spots after assembly.