spacefolder

Hi guys!

Compatibility question:

I´m in the process of rebuilding a 1996, Trek 750.
The original Bottom Bracket was quite grindy, so I ordered a replacement.
Couldn't find the UN55 around, so I went with the UN26 (as for what I read, this is basically the same BB, with plastic cup on the non drive side. Please correct me if I´m wrong).

Both being specd 68mm, BC 1.37 x 24 (btw, what is bc?), I placed my order online.
But to my concern, BBs are quite different. Please take a look at the pics.



Spindle size is 'almost' the same, but the "body" is quite different. When I install the BB, with the drive side fully screwed, I cannot completely screw in the plastic cup. 2-3mm stay "away" from the BB shell. Is this the wrong replacement?

Is this too bad? Any side effects?
Thanks guys..
Cheers!

fietsbob

All things being equal they will be

It just needs to match the thread on the frame and the over all length and the Drive side lengths to continue using the same crank

What it looks like is Not a functional criteria.

Did you get one for a 73 wide BB shell instead of one for a 68?

add a left side spacer, if that is true, so it tightens Up

spacefolder

Thanks fietsbob for the fast reply!

Thats's the strange thing. Both are marked 68mm, however a few 'groves' (2-3mm) dont fully screw.
I think that a spacer will do the job. Strange anyway. The pictures show quite a different design an fit.

HillRider

They are interchangable and you bought the right size and width. The plastic nds cup is just there as a stabilizer and, as long as it bottoms out on the cartridge body it doesn't have to be flush and don't tighten it beyond firmly. You don't want to crack it and you can guess why I know that.

"BC" implies British, more commonly known as "English" threads. The diameter is 1.37" and there are 24 threads per inch and the most common bottom bracket shell width is 68 mm. The drive-side cup is left-hand threaded and the nds cup standard right hand threaded.

Retro Grouch 

I'm thinking that 2 or 3 mm is a lot. I wouldn't be happy with that.

That nylon "cup" for the non-drive side is at least 2 or 3 mm wider than the metal one it replaces. The first thing that I'd do would be to inspect the inside of my bottom bracket shell for a cable guide screw or other obstruction. If it was my bike I'd probably make sure the main part of the BB was the same diameter (I'd be surprised if they aren't), and re-use the metal "cup" from your old bottom bracket.

dsbrantjr

That would be my approach as well; the cup is not a wearing item so should be pretty much infinitely reusable. Just be sure you grease/anti-seize the threads; I use Tef-Gel. If you want new, you could get one of these: Problem Solvers

ThermionicScott 

Don't go crazy with the spacer. With cartridge BBs, what is important is that the cup makes solid contact with the rest of the unit when torqued down, NOT that it bottoms out perfectly against the bottom bracket on your bike. (Although it's nice when both conditions happen.) Too thick of a spacer could prevent the two parts from joining up properly. HTH.

Trevtassie

What Retro Grouch says. I'll bet there is a screw for a cable guide poking through the shell and pushing on the side of the BB. It puts it out of alignment so the NDS cup can't screw in all the way or the cup hits the screw. (or since you've had a sealed BB in there already it may just be the cup too long) I've had both a few times. Either remove the screw and shorten it by filing or grinding or replace with a shorter one (which you're unlikely to find) Make sure the filed end of the thread is cleaned up nicely so it'll go in easy. When you are re-installing it wind it in like you are unscrewing it until you feel the thread drop into the start on the thread in the shell. If you had to, you could shorten the cup by filing as well. Do the same thing when you thread it in. Takes a bit of feel on the cup thread because it's really fine. This just makes sure you don't cross thread it so much... Usually the inside of the NDS cup has a slight taper.

spacefolder

Summarising, you guys agree I have the "correct" replacement for the BB?


1) There is no obstruction inside the BB shell. Checked.
2) I've tried to reused the old cup, but surprisingly the inside diameter is not the same.


3) Assuming this is the correct BB replacement, I think I'll take this approach. Just file or hacksaw some mm off the plastic cup.


Thanks for clarifying Hillrider!


Cheers!

spacefolder

Thanks Retro. I thought the same thing, and was just about to hacksaw / file those extra mm off from the plastic cup , when I noticed an inscription (very small print) on it. Guess what, it said 73. So I pedalled my way to the bike store , they apologized for the mistake and gave me a new 68mm plastic cup. Now everything is flush a nice!

Bottom line, whenever buying a new bottom bracket, be sure to check the plastic cup size too.



Cheers!