It might seem like an odd swap but my Ultegra BB is wearing out, I'm short of pennies and I have an SLX BB hanging about.
Any reason I can't or shouldn't replace one with the other?
--Twirlip
You won't end up with the right clearance for your sprocket.
Better to get the right BB rather than mess up your chainset.
I've done it the other way. I know they're spaced slightly differently but it was either bung the spare road BB cups into my MTB or not ride. So I put them in, adjusted the mech and made the ride. As it happens I left them in for a good few wweks without problem but did swap back to MTB BB cups once I got round to it.
I would be a [i]bit[/i] more hesitant putting MTB stuff on my road bike just 'cos the tolerance seem a bit tighter so it [i][b]may[/i][/b] not be possible to adjust for the different spacing.
To be honest, unless the Ultegra BB is really borked I keep it in 'til you can afford the right replacement.