Re: My frame broke.
Pic 3: Spacer out. The 3/8 inch threaded pivot rod passes horizontally through mount. I didn't want the steel rod to wear out the aluminum and get sloppy so steel sleaves (spacers) are used; 1/2 OD, 3/8 ID. They'll be J.B welded in. (Never used JB weld in my motorized bicycle before, but it seems to be a popular item)
Pic 4: Spacer in. As Ace Ventura would say; "LLLike a glove!"
Pic 5: Clamped. Skip forward quite a bit. Block has been "split" on band saw to clamp around seat stays. Spacers epoxied in. I let them extrude beyond the edge of the aluminum the thickness of a washer, for better side wear. Then the mount block was drilled and tapped so there are 4 allen-head bolts clamping it to the seat stays with a layer of inner tube rubber for damping.