Washers on motor screws
I does not matter what type of washer you use as long as they fit in the slot and the screw goes through the hole.
The obeject is to protect the aluminium from the chewing of the screw as it tightens and also to reduce the tendency for the motor to move as you tighten.
I use steel washers and they are fine. Do not use spring washers.
regards
Don