Michael,
As far as I am aware, Douglas used the same large bevel pinion on all postwar machines. The small pinion - whilst carrying the same part number - was altered on the later Dragonflies. It was nothing that would affect the interchangability of the gears - it was just the O.D. was reduced to clear the beefed up casting around the rear mainshaft bearing.
Whilst it was common practice to suggest that the gears should be changed in matched pairs - that goes for all gears, not just bevel gears! Having said that, Douglas only provided one adjustment for the mesh of the final drive gears (by shimming the output bearing housing). Normally, bevel gears have some adjustment for the small pinion as well. Without this adjustment, and no mention in the handbook on the correct procedure for shimming the large pinion, I would suggest the majority of surviving machines are running quite happily with less than ideal adjustment of the final drive. If it was on my bike, I would swap the damaged pinion, shim it up the same as the original, road test it, then by trial and error, adjust the shimming to get the quietest running.
Regards,
Eddie.
