Put a pair of pliers on the contact-breaker
centre-screw and check there is no detectable radial play, and no axial play
of more than a few thou. If the drive pinion/sprocket or drive shaft of the
magneto is accessible, do a similar check at that end. If there is play, it is
indicative of a bearing insulator disintegrating, serious bearing
problems, or that the armature wasn't shimmed correctly when it was last
rebuilt, in which case the magneto will need a more complete overhaul.