Or maybe not. But I see this question coming up every now and then and I heard different answers from different people. A guy at local bike store (big national chain starts with "P" and ends with "e") said "no difference" with straight face. He proceeded to say he orders road BB for everything. Mmm, ok... So I got a hold of XT BB and road (SM-FC4500, Tiagra) BB and did some comparisons. Disclaimer - I have no road bike, so I don't know if Tiagra is a decent grade or not.
In the following comparison pictures, left is XT, right is Tiagra. Road cups are slightly thicker than those for mountain.
XT inner cover:
Tiagra inner cover:
Left 2 are XT, right 2 are Tiagra - XT has more material inside the cups where Tiagra is CNC'ed deeply.
Both XT, left is right cup and right cup is in left (sorry). Obviously, right cup has more threads - therefore spacers are intended to go onto right (drive) cup, I think.
So my conclusions:
1) Rroad BBs, in Shimano's case, are meant to go onto 68mm ~ 70mm shells. Any longer (i.e., 73mm shells like most mountain frames), and inner cover will NOT seal one or both side of the BB since the road inner cover itself is about 5mm shorter than that of mountain.
2) If you want to use road BB for 73mm shell, then you will have to get a hold of inner cover meant for mountain, else your BB will get water/mud and will be destroyed.
3) Right cup on mountain BB has about 2.5 more pitches. Therefore, BB spacers (2mm, 2.5mm) are meant to go onto the right cup by design. Putting spacers on left could, in theory, compromise the BB integrity.
Or maybe not?
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