Shimano BB91 Press-Fit Bottom Bracket Originally designed in conjunction with Chris Cocalis for the Pivot Mach 4 frame, the Press-Fit mountain bike bottom bracket standard has been widely adopted by other manufacturers for its simplistic and lightweight design. By eliminating the alloy cups used on standard threaded external bottom brackets and simply locating the bearings directly in the frames bottom bracket shell, the BB92 standard not only increases frame stiffness, it also shields the bearings better from water and contaminants. The PressFit system works the same way as a headset; rather than threading in cups into the frame, the bearings are housed inside a plastic sleeve that is pressed directly into the frames BB shell. The beauty about this design is that it is still compatible with regular 2-piece Shimano cranks, such as the Deore XT and XTR models, and you don't require a special crankset as you do with BB30. This is because the BB92 PressFit bottom bracket bearings are still designed around a standard 24mm crank axle diameter. As for removal, once the bearings wear out, simply press out the sleeves and install a new bottom bracket as you would with a headset. Note 1: BB92 refers to the width of the frames bottom bracket shell; 92mm. This is not to be confused with the road version of this standard; BB86, which uses an 86mm wide bottom bracket shell. The different width is due to different chainline spacing between road and mountain bikes. Note 2: BB30 is a completely different standard. BB30 uses a 30mm crank spindle as oppose to the standard 24mm, and so requires both a BB30 specific frame and crank.
The Shimano BB91 Bottom Bracket Features:
Ok