Yes I get the rotor turned down to a .020” gap, ie .040” under size. I read this years ago in Stan Shenton’s Triumph Tuning book. I get both the rotor and stator measured and then remove what’s required for 0.040” undersize.
My Commando had definite signs of rubbing at .010”. I’ve not had issues at .020” gap.
I do this a ‘standard’ on all my old Brits now.
I got the bits on the Commando measured carefully after it rubbed to try and see if there was another root cause. The rotor for roundness and concentricity to the bore, the stator for roundness of the ID and concentricity to the mounting studs, and maybe some other stuff I’ve forgotten. Basically, it was all good. So it was going oversize with the gap that made the difference.
My thinking is that these old things flex more than we might realise, something that’s probably exponentially affected by rpm, so someone who seldom sees 5k rpm will be much less affected than someone who sees 6k+ frequently.
As an aside, I believe that modern pattern parts cannot be trusted to even give a .010” gap, I’ve seen them tighter than that, as have others. My hypothesis is that the manufactures think ‘tighter is better’ as they would on a modern machine, and perhaps don’t really understand the nature of old Brit bikes.
As to whether any electrickery is lost by having a bigger air gap, I don’t know. It would require someone cleverer than me to ascertain that. I‘ve never measured it and never noticed any ill effect.
I’ve also drilled out the stator mounting holes on a couple of occasions and don’t quite understand why I didn’t have any problems doing so, maybe my drill bits are nicely radiused (ie blunt) !?
