The back mount of my motor doesn't actually need the additional collet fixed to the shaft to act as a spacer. The shaft is fixed to the can and a combination of the magnetic attraction and a circlip at the front holds the motor together. I could run the motor with just the front mounting plate holding it on; the back mount is just to add support to try and reduce motor movement during impacts. So there's no lateral movement that needs to be restricted. I just added the second collet to the shaft because there was space for it, and to make it look better :P