How will you hold your aluminium plate in place?
well, not at all. as you can see in picture 3 in post #633717, the BSA-shop machined a recess into the timingside casing. probably to standard dimensions for the crank and to provide a reasonable flat surface to press the main bearing bush to. the depth of this recess is small, prob less than 1~1,5mm (1/16 inch) but enough to prevent the alu-plate from sliding forward and backward. the ends of the recess are round and the alu-plate is filled to fit preceisly. so the axial bearing fits snugly, free to move a little bit but not much.