I am looking for some advice on rear chains and sprockets. I am restoring my 1956 Norton Model 50 with one of Mike Pemberton’s 520 conversions. Mike also rebuilt the gearbox and fitted a 20T gearbox sprocket. Is this standard? Can't find any info in my 1956 manual. The original 43T rear wheel sprocket will obviously need replacing as will the chain. The original chain has 90 links and is of size 5/8” x ¼”. How many links should the new chain have? Still 90?
According to the '56 parts list 17T or 19T options were available. I assume that 17T was for sidecar use. Going up to 20 teeth will probably require a longer chain. As the increase from "standard" sprocket size is an odd number of teeth you may find that a cranked link is needed, depending on the amount of adjustment available at the rear axle. I would suggest getting the primary chain adjustment right first as the range of gearbox movement is fairly limited. If you could add a few links to your old chain you could thread that through and assess the correct length by taking out 1 link at a time.
If your rear sprocket is still in good condition, you shouldn't need to change it as a 520 is a 5/8x1/4".