In 1992, Shimano released their sealed bearing cartridge bottom brackets for use with their road and mountain bike parts groups, this is the "UN" series of bottom brackets. The three, and the added Dura-Ace 7410 model, have some variations in their appearance and weights. The quality differences in terms of materials used, tolerances manufactured to or bearings used are less discernible.
We called Shimano headquarters in Irvine, California to ask how they differ. Robert explained that the more expensive models are made of better steel and bearings. We asked how, what type of steel is used for each, and how or what type of bearings are used for each, so we could explain to customers why they should buy one over the other. Robert said that this information was "product confidential". When we asked if the spindle or bearings could be changed individually if they should fail, he explained that there are no user or mechanic serviceable parts in the UN series bottom brackets.
The assembly is warranted by Shimano for up to one year from the original purchase, a customer sending a defective unit to them would receive a new exchange for the defective B/B. For all other specifics he referred us to the Dealer's Product Manual, which frankly has nothing specific related to the components, or materials, though it tells available spindle lengths. All we can tell you is what we observe about each.
The bearings used on both sides of all three, are labeled "HIC" and "JAPAN". They, frankly, appear to be the same bearings in all three, and they all "feel" as though they have the same bearings. Ultimately, we ripped apart a UN-50, (the predecessor to the UN-51) to examine the bearing assembly and learned that the term "sealed cartridge", which is Shimano's term, isn't quite an accurate euphemism for "precision sealed cartridge bearing". The UN-50, when you removed the outer rubberized seals around an aluminum washer, revealed seven approximately 4.5mm diameter steel balls in a plastic retainer. With both outer seals removed, you can look straight through the B/B unit. These outer plastic seals, and their integrity, like those used in the Tioga MTB-SK, are all that keeps the outer world shielded from the inner 7 steel balls. The UN-51, UN-71, UN-91 and the DA 7410 B/B all outwardly appear to have the same bearings as the UN-50 we tore apart for inspection.
Shimano has created a special tool for installing these B/B's. The same tool is used to tighten the right cup, (which is fastened permanently to the sealed unit's core), as well as for installing and tightening the left cup into position. The Park BBT-2 or the Shimano TL-UN72 are the only known tools that securely seat the cups. The TL-UN72 was included with the UN-90 B/B, which was the UN-90's predecessor. As the Japanese yen has gained strength against the US dollar ,the included, seemingly free tool was removed to keep the B/B affordable. Buying or borrowing one of these tools will be requisite to installation under warranty.
The 1994 Shimano XT, LX, and STX Compact crank models are all Super Low Profile models. Super Low Profile triple cranks should use either a 113mm or a 118mm spindle instead of the traditional 122mm or 127mm spindle length. SLP double cranks, Dura-Ace or 105SC/RX100 should use a 103mm or 107mm instead of the traditional 115mm. Regarding oversized seat tubes, keep in mind that a larger diameter seat tube moves farther out to the right, away from the frame, on the drive side of the front derailleur cage. This will require the chainrings also be spaced farther away from the frame by using a longer spindle.
The included Shimano table shows the different possibilities of UN series B/B's with spindle lengths, shell widths, and weights, if we could source a specimen.
Now, the specifics about the B/B's as we see them.
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.
Spindle Length-07-13 $Price in Catalog