I changed out the bearings in an O.S. Hyper 50 today. When re-assembling, the threads in the engine casting where the backplate bolts on stripped out. It's not the bolt, but the aluminum threads in the engine casting itself. The other three torgued down just fine. I hand tightend all four then tightened them with an allen wrench opposite corners first, they did fine until the last one just kept turning. I backed it out and the aluminum threads are on the bolt. I didn't tighten that one any harder than the others.

Is this fixable? If so what is the best way?

I have torn down a lot of engines and never had this to happen.