Centre Bearing

Talk amongst many engine builders will throw up several suggestions as to how to fit the centre bearing in the 3 bearing Seven engine, and some will decry the use of any form of shims. However it was proven to me when Roger Spearman helped me rebuild my own engine that they are certainly necessary between the mating faces of the bearing caps. Notwithstanding the subject of alignment between all three bearings, which would fill several pages, I have assembled an old set of caps and shells sprayed black and white to show clearly in the photo where the shims are necessary.

When the two halves of the shells meet there is still a gap between the ends of the caps. Now imagine when the bolts are torqued up to the requisite 50lbs/ft – the shells will be crushed onto the crankshaft. Therefore, as a starting point you will need to measure this gap and make up appropriate accurately fitting shims – which won’t be right first time. In fact it could take several attempts to get the right clearance as you increase or decrease the shims by 1 or 2 thou at a time. You may find that at 30lbs/ft the crank spins beautifully and then at 35 it just starts to pinch, requiring another shim. You can imagine how many times you might have to do this. Incidentally, if say you have 5 shims of 2 thou, it would be preferable to change this for one shim of 10 thou.

Peter Lawson

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