Setting up old Brit Iron boxes, and some of the later unit construction ones too, was done by trial and error - the shims were increased a couple of a thou at a time and the end cover bolted down, until you could feel some resistance when turning the shafts. You then went back to the previous shim and that was it. A simpler bearing set up than airhead boxes made this possible. But, if you had a surplus cover, instead of machining the outside flat if you opened out the bearing recesses slightly until they were an easy slip fit over the bearings you could do the same on a airhead box, one shaft at a time. You would have to adjust the shim stack to suit the difference between the pocket depths on the trial and the good cover, of course, but that would be the only measurement required, so the possibility of error is greatly reduced. Anyone see why this system wouldn't work on a airhead box.?