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-51 is the successor to the UN-50 and least expensive of the UN bottom brackets. One obvious improvement Shimano made to the UN-51 was to upgrade the left bearing cup from a grey plastic part, which stripped and cracked, to a stamped and machined steel part. The central core of the "sealed" unit, which retains the bearings, is made of fairly sturdy steel. The exterior of this central core is smoothly turned, with the right side bearing cup pressed permanently onto the right side of the core. The right cup is made of machined steel that is colored Silver.
The spindle on the UN-51 appears to be made of cast Black steel that is turned on a lathe to perfect the outer dimension. It is solid, with the ends bored and tapped for the crank bolts to screw in. The left bearing cup is made of Silver colored steel, with the threads and tool fitting end machined into the piece. The UN-51 comes with a pair of one piece crank bolt/washers made of Black colored steel.
It is available in English threads only, in either 107mm, 115mm (for Road double), 113mm, 118mm, (new LX and XT), 122.5mm (for Road or Mountain triple) or 127.5mm (for Mountain triple) spindle lengths. The UN-51 using a 122.5mm spindle to fit a 68mm B/B shell, with the included crank bolts, (which weigh 30 grams), weighs 358.5 grams. It is the specified B/B for the 105SC-7, RX100 Road and the Deore DX (discontinued in 1994), LX and 400 CX Mountain parts groups.
The UN-51 does not come with the necessary TL-UN72 installation tool. The U.S. made Park BBT2 is recommended and found toward the end of this section.