i’ve got two frames which i swap parts between (rocky mountain element and spesh enduro). The rocky has a 73mm bb, the spesh 68. Fine with a ht2 chainset due to spacers but i’m considering using a square taper again and that’d mean two bbs. So any issues with facing the rocky down to 68mm. There’s plenty of clearance and the bb threads are deep enough.

just buy two bottom brackets 😕

Are you sure you want to take square taper cranks on and off? Each time you do that does it not make them travel further down the taper?

mmm,remove 5mm of material with a facing tool, you are joking I hope..

It’ll balls your chainline up

5mm of material removed with a facing tool isn’t going to be doing the tool any favours either.

Its done on some of the 83mm DH bikes so lighter cranks can be run.. 10mm off!

Just use a bottom bracket like the Raceface that doesn’t use a shoulder, then screw it in or out as far as you need.

Still dont understand why you want to swap the BBs over. Just buy two? One for each frame, thus saving a lot of faffing and maintaining the chainline etc.
Bear in mind the BB is that width for a reason, not just because they couldnt be bothered to chop a bit off the BB shell!

I got some UN26 BB’s for £5.50 each, and they seem to last just as long as more expensive ones (6 months so far and still no play)