Press-fit bottom brackets get a bad reputation, and most of that reputation comes from installations that were rushed, frames with sloppy tolerances, or bearings that weren't sealed well enough for the conditions they were asked to handle. Shimano's BB-MT800-PA is the XT-level answer to that reputation—a fiber-reinforced polymer cup design built for HollowTech II mountain cranks with 24mm spindles, sized for 41mm ID frames in the 89.5mm to 92mm shell width range. That covers BB86, BB89, BB91, and BB92 frame standards, which is most of the press-fit mountain frames coming out of the major carbon manufacturers.

The composite cup construction is doing more work here than it gets credit for. Resin cups have a little more give than aluminum when they seat into a carbon shell, which reduces the risk of creaks from micro-movement between two rigid surfaces and lowers the odds of damaging the frame during installation. Shimano packages 2.5mm spacers with the bottom bracket so it can adapt between 89.5mm and 92mm shells without swapping parts—useful if you're building up a frame and haven't measured the shell to the tenth of a millimeter yet.

Sealing is where the MT800 generation earned its keep over the older SM-BB71/BB92 units. The updated seal design keeps water and trail grit out of the bearing race without adding meaningful drag, which matters because mountain bottom brackets spend more time submerged, sprayed, or caked than any other bearing on the bike. Pair that with the standard Shimano preload adjustment procedure—snug the star bolt just enough to remove lateral play, then let the crank arm pinch bolts handle retention—and this is a bearing that survives seasons rather than months.... Read More