I had a similar problem, on hard impacts the motor would move away from the spur causing it to strip. The issue was I was using a Feigao 8XL and the endbell aluminum was very soft so I could not really torque the screws down the way I would normally for fear of stripping them. I temporarily got around this by adding a little tab to the motor mount on the outside edge, this tab had a screw that pushed the motor towards the spur and kept it fom moving away from the spur. It worked fine, until I finally got a better motor (Neu) where I could torque the screws properly to keep it tight and no longer needed the extra tab. In the picture you can also see the bracket I made to hold the Feigao endbells on.
