>>1851539many older mtbs and hybrids still have 68mm road shells, not 73mm.
this is a slightly complicated area though.
With hollowtech II, in theory, 'mtb' 73mm bbs should each be 2.5mm narrower than 'road' 68mm bbs. And then they both fit the same cranks on mtb/ road frames because that 5mm shell difference is accounted for by the bb.
Pic rel shows 2 that I measured, left and right are the same bb, left has a 2.5mm spacer on it. In reality though, not all 'mtb' bbs, even shimano, are 2.5mm narrower. And not all frame shells are an exact width. Complicated by the fact that many of them are or should be faced to remove material. I have found in the past that mtb cranks on road frames with road bbs still needed spacing, and that road cranks on mtb frames didn't have enough engagement. It's slightly fucky.
The way the preload works on shimano cranks gives you some leeway here though. You can space things, and you can get a shop to face some material off the shell (that's actually a good idea anyway running newer cranks as they will make the surfaces flat and parallel and you'll get better bearing life).
>what is the best solution here you can run a road crank, on a mtb bb, and maybe face the frame some. In most cases I run a square taper crank because it's cheaper and looks better and has a longer service interval. Especially if you're planning a chinkshit chainring, but still ona setup where you drop hundred bucks. Bit of a waste. If you want a large chainring, 130bcd hollowtech II 105/ultegra cranks from 10spd era look better and are very cheap used parts with thrashed rings.
