For your entertainment and delight, this very Forum has all the parts catalogues you need, right there in the Literature Section. Scroll down until you find a post by esteemed member Sluggo, and in that thread look for the early bikes' part catalogues posted by JulianS. These latter are a little different, but it is all there. The splines on the rear hub and the bolt on flange on the drum are the same for all models and years as far as I know, all the way from late A7 Longstroke to A65, all models with crinkle hub rear wheel.
Rear brake drum and sprocket for rigid and plunger comes in 42, 46, 49 teeth variants, but also offered with different width teeth to suit narrower chains used on lower powered models within the BSA Model Range. ebay sellers lump them altogether, so care needed if buying a used example. It is also easy to confuse with the early S/A crinkle hub type drum, here the centre hole is smaller, as is the ballrace. Condition of the teeth is most important, the drum can be skimmed, but a sprocket with worn teeth is expensive to recondition.
A7 Gearbox sprockets were 19 teeth as standard, with a 46 tooth drum. 18/49 was available for sidecar use. Hooligans can now use 21/42 thanks to aftermarket upgrades. For a standard bike it would be 19/46, but 19/42 gives a good all round solo performance. 20 or 21 may be a little high for a 500, OK on a 650. All rigid and plunger standard boxes have the same internal ratios.
Swarfy
