Shimano BB MT800 Mountain Bottom Bracket. The Shimano BB MT800 bottom bracket is the XT level bottom bracket designed for Shimano's new 11 speed XT mountain cranksets. The MT800 bottom bracket benefits from technology already in place on the flagship XTR edition, and the trickle down means this bottom bracket is lighter and has less bearing drag than the previous XT model. Not only that, there's improved durability via updated seals to ensure mud and grime are kept out. This version is for English threaded bottom brackets.