Never having done this, and not knowing the exact configuration of the brake drums....
I'd be inclined to make up a special faceplate or other fixture to mimic the way the drums are mounted on the motorcycle. Then bolt them in place as they are normally bolted in place, and machine. I have a hunch that trying to set them up in a 4-jaw would be a PITA and not very satisfactory.
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Everybody is ignorant, only on different subjects. -- Will Rogers
There are lots of people who mistake their imagination for their memory. - Josh Billings
Law of Logical Argument - Anything is possible if you don't know what you are talking about.