BadBurrito

I'm not a professional bike tech, I just work on my own bikes

I'm rehabbing an 1997-1998 giant road bike. the existing bottom bracket is a shimano BB-UN53 68 X 113.

I can't find an exact replacement, but I did get a shimano BB-UN55 68 X 113, I thought it would fit but I can not seem to get it threaded on.

shouldn't all 68 X 113 be interchangeable?

Is this new BB just a bad substitute? or just bad (as in it should work, but this particular one has bum threads or something)

What is a good replacement for the BB-UN53?

Looking for recommendations

thanks

markjenn

You probably know this, but just in case, you realize that the right side (which goes on first) is reverse-threaded?

- Mark

lopek77

+1

and don't force it in, in any way...threads can be damaged, wrong or just dirty.

mrv 

did you read through the Park Tool repair section? Park Tool Co. » ParkTool Blog
maybe - possibly - you might need to run the frame up to the local bike shop to chase the threads.
it's easy to do, but the tool is like $500, and you don't want to do it wrong. ask if they will show you how to do it for $20~$30. you can learn something and get the threads cleaned up.
my LBS let's me 'rent' tools, and the great thing is the bike shop owner is there to prevent me from doing it wrong.
- another Mark

BadBurrito

yeah, I do know it is reversed threaded on the right side. I appreciate the response because you sort of imply that it should fit if only I put it on correctly. That is a help.

I should add, it a square taper spindle.

also, just for my education. there is another set of numbers that is BC 1.37 X 24 - what do these numbers mean. I was wondering if the 24 was threads per inch?

Bill Kapaun

1.37" (1-3/8) by 24 TPI.

IF the old one screwed out OK, the new one should screw in.
Since it's a fine thread, what I do is-
Line it up "square".
Rotate backwards until you FEEL a thread kind of "drop in".
Screw in by hand, making sure you aren't cross threading.

Clean the BB threads before installing. Something like a tooth brush, preferably with metal bristles.
Grease threads or use anti seize.

hueyhoolihan

can you thread the old one back on? any particular side giving you trouble? if not, the threads may have been damaged upon removal. you might look closely at the two and compare thread patterns and diameters.

BadBurrito

the old BB goes back in super smooth, very easily, I've cleaned the threads on the frame just with a cleaner and rag, I think I'll work on them with a toothbrush.

I compared the threads on old BB to the new BB. My eyesight isn't the greatest, but it looks like the first few threads on the new BB appear to be flattened a bit. So... I'm thinking my many attempts have mucked up the new BB's threads.

I'll get a new, new BB after I clean the threads better and be super careful the next time

thanks for the help

Bill Kapaun

Maybe the new one got "tossed" in shipping and was bad when you got it?
IF you have a small triangular shaped file, you might be able to clean up the threads enough if it's just a couple.
You should still have plenty of threads left to be secure.

JohnDThompson 

+1 this. If cleaning the threads with a wire brush doesn't help, only then consider chasing the threads with a tap.

hueyhoolihan

if the threads in question on the new BB are right hand and slightly damaged, and you have access to a BB BB lock ring you might be able to thread it on the BB from the back side and over the starting threads, using it as a poor man's thread chaser. i know, chances are slim...

JohnDThompson 

An old fixed cup can be turned into a thread chaser as well, by grinding some slots for the debris to fall into:

Bill Kapaun

The problem ISN'T the BB shell. It's the new cartridge.

Retro Grouch 

This^^ Also, it's easy to try to thread it on when it's not quite square. Sometimes it takes me a couple of tries before I get it right.

Amesja

The threads are really easy to bugger up. Many (most) bikes are not prepped very well and the bb shell threads are very rough and filled with paint from the factory, grit, bits of metal left over from when the shell was first tapped and just left hanging, and generally in crappy shape. I've got my own chase/face tool and properly prep every frame I have the bb out of unless it is obvious that the last "professional" mechanic to work on it was the rare non-hack.

Because of all of that it is REALLY easy to get that first thread crossed in a bb install and it will either bugger up the first thread in the shell, the first thread in the bb cartridge, or both. It will keep getting worse and worse and a mechanic will need to be patient and keep trying to back out the thread and find the engagement "click" that was mentioned earlier. I've done many of these jobs and have my own threader and still there are times when some bike just have a lot of fight in them. If you get "rammy" and try to force it then things will sometimes end in a lot of tears.

A thread file helps sometimes too.