Bottom bracket materials aluminium steel series shimano deore xt applications touring mtb construction shimano hollowtech ii weight 93 g housing widths 68 73 mm bearing position outside inner diameter 24 mm component recommendations shimano fc t661 shimano fc m665 shimano fc m660 shimano fc m785 shimano fc m780 shimano fc m985 shimano fc m980 thread bsa 1 37 x 24 eng