The shims are really there to take account of machining tolerances and to adjust the mag drive gear mesh as required...They are suitable for very light wear of the platform but no more...If the mag platform is badly worn I machine it flat (clean), removing one standard sheet metal gauge thickness as required...18 or 20 gauge will often be enough...It's then simply a case of cutting another 'shim' from the heavier gauge material to restore everything to its correct relationship...The outer end of the mag platform can be used as a setting up datum as that is never worn.. The mag base doesn't contact roughly the last half inch of the platforms surface..This procedure avoids the inherent risk in welding older castings prior to machining....Regular checking of the mag strap bolt is the answer but if you do inadvertanly let it loosen off the thicker shim plate will prevent a repetition of the damage...Don't forget to check the base of the mag for wear as well...Ian
I have a M21 where BSA machining tolerances were not very good. The bike died at 1200 miles from new as the magneto bearing insulating cups had disintegrated. They had machined the magneto platform too much and with the strap tight it put a big side load on the bearings as there was no gear clearance. There was absolutely no wear on the magdyno or platform due to the low mileage. It took a .016" shim to get the gear clearance OK.
I have seen many bikes (not just M20's) where the bottom of the magdyno and the platform are so worn out it required welding. No one ever seems to check the strap bolt. I have a Goldstar engine where they used epoxy to make sure the magdyno did not move. It was not easy to get apart.
Yes, in some cases welding is unavoidable though I try not to if possible...I've had wartime crankcase with an..err.. novel material mix that just could not be welded....Not only is it a moments work to check the strap bolt as part of general maintenance but the magdyno can clearly be seen moving about when the engine is running if the strap is loose and the chattering/whining of the poorly meshed gears is clearly audible...
I guess some people must have their minds on other things....:worried: Ian
Just sayin'
I once saw an M20 with the magdyno so loose it was jumping up & down on its platform to the point I reckon it was in danger of jumping out of gear.
