My BB shell is about 70.5mm wide, so using a 68 or a 73 BB would theoretically screw up my carefully planned chainline.

I know that 70mm is the shell width for Italian bottom brackets, but this is a 1980 or so Raleigh Grand Prix frame that appeared to still have the original BB and crankset in place when I got my hands on it. I figured I either had some weirdo BB, or the shell had been faced in at some point in time. Neither of which made much sense.

I found while installing the non-drive side cup for the new Shimano BB that the threading on that side and that side only is not standard. It is really close, as I could thread the cup in about halfway before the threads started to bind up, but when I held the old adjustable cup alongside the new cup, the thread pitch did not match.

I probably should have taken the BB back to the bike shop to see if there was a proper part I could use, but I wanted to get this bike rideable ASAP, so I drememed off half of the offending threads on the cup, allowing me to thread it in with only slight binding and still leave enough threading to adequately support the BB cartridge. Now that I see how off the chainline is, maybe I should have left the BB cup in a condition that had half a chance of being accepted for an exchange... Oh well.