So, after about 20 years of being a dream in a bunch of crates that I have moved at least once, I am finally getting to the reassembly of the BSA. I have the engine on its side, off of the floor with some 4x4s and am putting the frame around the engine.
I had the frame straightened by Wasco, and then powdercoated. I have cleaned the engine up, and polished the cases, and now am reassembling.
I bought new mounting bolts. The frame fits perfectly around the upper front mounting bolt. Slides in place with no side to side movement.
The lower mounting bolt has a gap in the clutch side (up in the picture), but no gap in the points side. I have the original mounting gear, and there was a spacer in there, which I have also attached.
I assume that the spacer needs to go in where the gap is (it fits - very tightly) before I tighten up the frame, but wanted to confirm this, as it has been 15 years since I pulled this apart.
When the frame is centered with the front bolt in place but the nuts not tightened, there is very little lateral movement - the front fits perfectly - so I can't shove the frame around to increase or decrease the gap shown.
Yes, you have found the spot for the famous and sometimes mythical-seeming "Packing Piece". The gap IS there and that spacer IS meant to fill it up. If it's not, the bottom mount with either try to warp the frame or leave a gap.
Why it was designed that way, perhaps others can speculate ...
Sounds like you are on the right track to me. The spacers should have a tight fit too keep from flexing the frame and causing undue stress on the frame which can of course possibly cause a crack and definitely increase vibration.
Thanks all - I'm going to follow your suggestions, and then conduct a final alignment when the rear wheel and sprockets are installed. That spacer has enough thickness that the frame wont be sprung when tightened, so I will install it where it best fits when the other mounting bolts are snugged, and then when the rear wheel is installed, remove the lower bolt and shim for sprocket alignment.