False teeth
Unable to obtain a new rear sprocket for his 1928 AJS K8, John Scott had it re-toothed by a local engineer – but he now finds that the sprocket is slightly eccentric, leading to chain tension problems. ‘Do you know of a company with the experience to do this properly?’ he asks.
‘JOHN’S SPROCKET IS PART OF THE BRAKE DRUM’
This is very difficult. It’s tricky (but not too difficult) to turn down a plain sprocket and bore out a new one to weld to the original mounting centre – but John’s sprocket is part of the brake drum. This means that a toothed ring has to be fitted onto the brake drum; being little more than a hoop, it has little structural integrity and the heat from welding it in place is likely to distort both it and possibly even the brake drum itself. That being so, it would be much better to bolt on the new sprocket. Machining back half the thickness of both old and new sprockets will create a flange where they can be joined, preserving the chain line... in theory. Trouble is, usually the sprocket is not much larger in diameter than the outside of the brake drum, so the bolts wouldn’t clear the chain. One solution is to secure the ring with grub screws. Remove the old teeth and bore the new sprocket to a press fit, then drill and tap several holes around the join, and screw in hightensile grub screws. These will prevent the sprocket rotating or shifting out of line.
You could also increase the size of the sprocket, creating more space, and maintain the gear ratio with a larger gearbox sprocket. Who would I ask to do it? Not sure about that, but find an engineer experienced in old bikes because not everyone understands the complexities of an apparently straightforward job like this.