The specs say 68 or 73 BB shell but I had problems using this exact model on an older trek-850 (with 68 shell and 130 rear drops). There were not enough spacers to get it installed with the correct and balanced extensions on both sides. I set it aside and ordered a road crank (tiagra I think) because I wanted the best Q. I have since installed it into a 73 BB shell and it fit perfectly.