Bottom bracket material aluminium series rotor pressfit application racing bike constructions bb71 shimano hollowtech ii weight 74 g housing width 86 5 mm bearing position outside inner diameter 24 mm component recommendations shimano fc cx70 shimano fc cx50 shimano fc 3503 shimano fc 3550 shimano fc 4603 shimano fc 4650 shimano fc 4600 shimano fc 5703 shimano fc 5750 shimano fc 5700 shimano fc 6703 shimano fc 6700 shell diameter 41mm mm