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Assemble your cases (without the crank) with the bearings installed. Measure the distance between the bearing faces.
Measure the width of your crankshaft (the part that will touch the bearing faces).
Subtract the crank width from the bearing distance.
Divide that result by 2.
That is how thick your shim is supposed to be.
... and the shim goes on the PRIMARY side, not the magneto side.
Just to confuse ya all---unless i'm confused-- there should only be a shim on the righ side as the crank is bolted tight over that way by the primary hear nut. Anything you put on the mag side will do nothing except flop around although if you calculated it absolutely exact it could be under slight pressure to keep it from free wheeling. Ok that's my idea & now i'll go & look at a parts pic.
Ok, to Edit, had a look link below & nothing on the mag side. Granted the pic is a bit confusing but follow the line around to the right side, primary side, & No24 says 0.3mm thick or 12 thou by my calculations. Realy the idea --i think--is to end up with the conrod as close to the middle--crankcase joint--as feasible