Originally Posted by sydney
My guess is he is talking 68 vs 73mm shell width.
That would be my guess to. That suggests that he's trying to transfer parts from a road bike onto a mountain bike frame. Cable routing, chainring clearance, and front derailleur clamp diameter are all possible incompatability issues that pop off of the top of my head. I just think that I'd be more likely to make an accurate guess if I had a little more information.