If I were to attempt to put an MTB crank and BB, designed for a 73mm BB shell, into a gravel frame with a 68mm BB shell, is everything going to burst into flames I just add a 2.5mm spacer to each side of the BB? Or thinking about it, to the NDS to preserve the chain line?
I am going to fit a properly fitting crank and BB eventually, just not right now.
Yeah, mtb crankset for a 68/73 will drop in no problem with spacers. I sometimes run an old deore triple on my gravel bike by doing just that. It will need the mtb bottom bracket though, road/gravel ones are narrower.
The front mech however, is an entirely different story
Front mech is being banished 😊. Ta!
BSA road bottom bracket is 91mm face to face, mtb 68 or 73 is 96.5. so long as you make up the difference, or close enough, it will be fine.
The Hope BSA 68/24 road bottom bracket has a 1mm spacer on each side. Use that same bottom bracket for an mtb and it's 2x2.5 spacer drive side and 1x2.5 disc side. In a 73 shell, the mtb bottom bracket is the same width with just one 2.5mm spacer on the drive side.
