SHIMANO UN-91 B/B
The UN-91 is the successor to the UN-90 and made specifically for the XTR and Dura-Ace crankset, because the B/B's shorter spindle length and of how deeply the arms recess over the spindle toward the bottom bracket shell. Shimano suggests that combinations other than these cranks are not possible for this B/B. The center core of the UN-91 is made of steel. Between the bearing cups, the core is turned on a lathe to reduce the diameter, and the weight of the unit. The right bearing cup is made of Silver anodized aluminum and pressed permanently onto the right side of the center core.
The spindle is hollow, made of a silver colored steel, like the UN-71, with the ends tapped for the crank bolt threads. The spindle appears to be cast because of the appearance of it's 2 degree tapered sides, and is then machined after to perfect the surfaces. The left bearing cup is made of machined aluminum that is broached on the end, to create the fitting for the TL-UN72 tool.
The UN-91 doesn't come with crank bolts because the XTR and Dura-Ace cranks come with the necessary crank bolts. Using this B/B with other cranks will require these bolts, a selection of which is found in the crank section of this book. As mentioned in the UN overview, the UN-91 does not come with the required TL-UN72 tool. The U.S. made Park BBT2 is recommended and found toward the end of this section.
The UN-91 is available in English threads only, in either 107mm or 113mm spindle lengths. The UN-91 with a 113mm spindle, weighs 239 grams.
