One small addition to Dave Bushell’s post, the BTH magneto, like the Lucas, is taper mounted and has a key in the taper to positively locate the breaker assembly on the armature at the correct relative angle.
The point here is that on several occasions I have come across mags where the locating key has either broken off or has been removed. It seemed to mostly happen on machines with an ATC unit because it was it easier to rotate the breaker assembly on the magneto armature than to be on both sides of the bike at once, needful because Lucas in their wisdom were inconsiderate enough to put the ATC on the drive side and the points on the other!
It took me a couple of thousand miles on a £35 Vincent Comet that I bought as a hack, (them were the days!). I got it running with a minimum of attention and only found the missing key when I got around to doing a proper service.
So, just in case, make sure your mags contact assembly has a key, yer never know!
