Hello Richard
I assemble the crank into the cases and measure endfloat with the outer bearing race bottomned in the primary case half, all screws, nuts, bolts clamping the cases together - minus sealer. Then with a dial gauge set up at the cranks end the endfloat is measured. Lets say it is 0,25mm and we want only 0,05mm it would mean I insert a 0,2mm washer behind the inner race or the outer race of the primary side bearing. If I had the correct washer made of suitable material I would be finished here - but I have no suitable washers as I mainly do Norton and Vincent engines. So I made a washer. The reason I use a thicker washer and grind the bearing down a bit is due to my grinding machines not holding a thin shim on the magnetos - it will just fly away or if it stayed on the magneto it will become a bellville spring due to internal stress. So in our example the washer is 0,3mm too thick - this is the amount that I grind off the bearing. As the radius on the outer race of the bearing is a lot larger than the radius of the recess in the crankcase there is no reason to fear the bearing and washer behind will not go fully home. As I said before - even lose races on the crank with some glue and some clamping force of the primary nut will hold but the amount of transferable output will be reduced and once the shim behind the race is collapsing the wear starts as there is no clamping force at all. You will first notice that something is amiss when you hear the knock when the lose bearing hammered the crank undersize or when the splines on the crank are worn away a good bit - both needing costly repairs.
cheers, Harty