The BSA tool for this job was a jig comprised of a plate with holes machined accurately that corresponded to the exact centres of the crank/idler/cam bushings,fitting over the crankcase dowels (part no.61-3281).It was bolted to the casings and provided a guide for the reamer (another BSA tool:61-3167) to do its job without losing the accuracy of the hole centres so as to maintain correct meshing/running clearence for the gears. Service sheet no.215 gives details .Finished bush dia is .7495"-.7485" according to BSA.I once saw one on US E-bay for sale, rare item ony likely to turn up when a dealership closes down, no idea if anyone makes aftermarket ones.Bob.