As the title really.

I have a spare brand new 10 speed Ultegra BB and need a new BB for the mountain bike.

The mtb is a 73mm bb shell can I use the road bb? Will I need to use the standard 1 spacer on the drive side?

Or is the spacing different so I’ll need more or less.

Help me save some cash?

Road BB’s are spaced for 68mm shells. And iirc, the cups are slightly narrower too (though I may be wrong there).

They’ll fit, but they’re slightly wider than mtb ones (cant remember if it’s 1 or 1.5mm each), so spacing will need to take that into account.

Narrower or wider?

I’ve measured them in the past just to be sure, road cups are slightly wider (and don’t run with spacers).

So if they are slightly wider then on a 73mm shell I prob won’t need any spacers at all then.

As usually you would need 1 spacer with a mtb bb

aye, no bother.
had a 105 one on a dmr trailstar for ages and it was fine.

68mm shell fits fine, although no spacers as a MTB would have.
73mm will work, but the inner sleeve won’t connect. It means its less resistant to water/dirt ingress.

Measured and fitted last night – 73mm bb shell and seemed to work well with xt cranks and 1×1.5mm spacer rather than a normal 2.5mm