Bottom bracket BB UN50 replacement
 

I've torn down and rebuilt a number of bikes, but in the cases where I needed to service/replace the bottom bracket and/or headset, I've had my local bike shop do it for me.

I recently rode a bike I have at a vacation home and heard some noises that I'm reasonably sure are coming from the bottom bracket. I'd like to replace the BB as I'm sure it's original. The bike is a 1992 Trek 8900 MTB. It has a M900 series XTR 8-speed groupset on it. Crank is the original XTR triple (46/36/26). Per the spec manual the BB is "39/52/40.5, 3UT Shimano BB-UN50" with a 68mm shell. The spindle length isn't known..the tech manual doesn't indicate it and I haven't taken it apart yet.

What more modern (or at least available) replacement BB will fit the bike? I assume whatever replacement is available will have stock spindle lengths available once I get it apart.

thx..

1.. buy a Crank remover if you don't already own one.

2... remove the cranks from the spindle and measure it.

or wait for someone here to guess at the length.. i just found FOUR different length BBs for that groupset, in a quick search of the non-specific group info you provided.

if you provide an exact Part number from the crankset, someone can possibly find the length that you could have already found with a fairly easy measurement.

ran into this thread along the way.... PS.. a 118mm version of the BB-92 (Shimano's Specified BB for that groupset.) is also available online.... they are all expensive.
https://www.retrobike.co.uk/threads/...onger%20length.

i strongly advise pulling your crank. :thumb:

also found along the way.... read up. your noise is probably not the BB actually failing... unless the bike was ridden to death, or neglected and left out in the weather for years... PS.. i've had luck adding a bit of extra oil to the SID forks to help end the bottoming out problem...

Bike Radar Reviews...
[Jul 08, 2002]
Tommy
Cross Country Rider
OVERALL
RATING 4 ★★★★★
★★★★★
VALUE
RATING 4 ★★★★★
★★★★★ Strength:Very light and responsive. Easy to manuever through technical singletrack. Hayes discs work great.

Weakness:Icon stem, bars, and seatpost. Bike creaks a lot, but I was able to fix it. Not very impressed with the sid so far after coming off of a 100mm manitou x-vert. I guess the shock is lighter, but it doesn't perform like a manitou. I'll give it a few more months and see.

So far I like this bike. It handles the singletrack nice. The sid is a little unforgiving, but it's very light. The only thing I didn't like about this bike so far is the awful creaking coming from the frame. I was able to fix all the noise pretty easily. If you have the same problem listen up. The seat tube has a shim in it that comes loose over time. You need to get green loctite [bearing and bushing] retaining compound to fix the problem. I think the number is like 640 or 680. Makes sure it's not threadlock or it might not work. It specifically has to be retaining compound. It gets into any gaps between the shim and fills them. First take the seat post out. Dribble plenty of loctite compound down the seat tube. Let it get past the shim(about 3-4 inches). After it passes the shim, turn the bike upside down to let it work into the gaps. Let it sit this way for a day or two. Turn the bike over and put the seat post back in. The noise should be gone. I also noticed the cranks and BB creaking. The cranks just need to be tightened and the BB was loose too. So far the bike has been beaten up pretty good on rocky singletrack and it's still quiet. I've only had the bike used for a couple of weeks and I've had to do these few things to it. It just needed some tweeking to get it set for my riding style. The times I had it out I loved it and the hayes brakes are awesome...

[NOTE: Remove the Bottom Bracket BEFORE adding any Locktite to the Seat shim... Clean any Green Locktite that dripped down to the BB area and threads BEFORE re-installing the BB. Also clean it from any inner surface of the Seat post shim...Green Locktite is a PERMANENT Glue, intended for retaining bearings and bushings in Engine and Transmission cases, Under Extreme Load situations.. and it works for that wonderfully....until rebuild time.. then you begin to hate green locktite.]

Thanks..yes I plan to do that, however at the moment the bike is 1300 miles away. It's the bike I use at a FL condo when I'm there. I won't see it again for 8-10 weeks. At this point I was just doing some research to at least identify a replacement BB..for example is a UN54 a replacement for a UN50? How about a UN52 or UN55 or UN300? The spindle length is yet to be determined, but at least I can understand what BB I'll need and the tools associated with removing it.

Of course there's always the possibility that someone here has a similar 8900 and may have knowledge of the bottom bracket used.

The bike has an original ridged fork so I'll have no issues related to a suspension fork. Prior to riding it a few days ago(I was doing a wellness check on the condo ..the bike has been sitting unused inside the condo at 80 degrees for the last 4 months) the bike has been completely quiet. It may be a pedal and that's easy to evaluate once I'm there with a pedal wrench. The bike in general is like new and hasn't seen typical MTB use on trails. I did a drop bar conversion on it and it's basically a road/light trail(Florida)/errand all-rounder at this point. I run 26x2 Schwalbe Marathon Supremes on it..it's a fun bike.

I know there's still plenty of square taper bottom brackets available, both good condition used and new ones, just make sure you get the correct taper (JIS or ISO). I don't have any model numbers off hand to give you, I've always gone down to the local recyclerly, bike shop I'm friends with or my parent's place to grab a good used bottom bracket that fits.

Before you do that though, pull the cranks and spin it. If it feels smooth, you may just need to thoroughly clean the mating surfaces between the crank and spindle, reinstall and then tighten the bolt. I've had bad luck with square taper crank bolts loosening, causing the cranks to creak and sound like a failing bottom bracket. I like to use a bit of blue loctite on the crank bolts and that has solved the problem for me.

i suggest that you read the links i posted earlier, and also read this link..

https://bicycles.stackexchange.com/q...aper-cartridge

the UN-50 specified by Trek is a compromise unit meant to cut the cost of the bike... and the LENGTH is mostly determined by your frame's design. Emailing Trek may be the only way to know what length they installed in your frame for that particular crankset... Another possible way to find a possible length would be to Look Up the Part Number of the crankset on the bike and hope Shimano has recommended a specific length for a typical MTB.... the kicker being that your 8900 is NOT really a "typical MTB".

there is not much rhyme or reason to the UN designations, other than to identify their Model class.. the usual groupset BB for your XTR parts Was a UN-91, if i recall correctly... the spendy one... there's a NOS one on Ebay right now for $250... and some others too.

And i can't stress this enough.. you might NOT NEED A Bottom Bracket assembly..... those bikes have other "noise" issues described in one of my earlier links.

i'm out..

Pretty much any Shimano UN series or Tange LN series english threaded 68mm of the correct spindle length will work.

i'll drop this bit of info here before backing away from this thread completely.

https://bicycles.stackexchange.com/q...dle-dimensions

good luck with replacing a part that may not be your "noise".

..well..there you go. Simple answer to a reasonably simple question. Thank you DH.


You're right and I knew that before initially asking the question. We also know that tracking down noises is often a process of elimination. I'll check the pedals by replacing them with some I have on the shelf. If that isn't it..I'll check the seatpost, if that isn't it, I'll pull the BB, clean up and grease the entire assembly as Pantah suggests, if that isn't it, I'll replace the BB (as long as I have it apart, I'll replace it as a preventive measure as it's 31 years old), if that isn't it..I'll try something else, but at least I'll know what it isn't..it's a good start.

I appreciate all the links and info you've provided, I really do, and I'll study it closely, however the drama surrounding the info is beyond my understanding. Thanks for your help.

EVERYTHING bicycle is dripping with drama ! !