Bottom bracket fitting problems, help needed

[Tootyred]

Could the bb be damaged? It will probably be just as it enters the frame..so if it goes in a few turns, mark the thread against the frame with a sharpie, take it out and look there....i bet you have a damaged thread that is stopping it picking up with the frame thread.....

Often, just a small file is all thats needed to clear a damaged thread on the cup.

Or just get another bb as you said

Good luck. These things are bloody annoying!

 

[RetroDavy]

I've no killer solution but have experienced the exact same thing: UN5x wouldn't go whereas a BB with separate steel cups went in fine.

Threads appeared ok. I couldn't work out if the BB shell might have been a fraction out of round.

If it had been a frame I was attached to and really wanted to fit that cartridge BB I would have got the frame threads chased out.

 

[Once A Hero]

@ishaw is this the BB I sold to you?
Apologies if it is!, I think we both agree it's in very good order - so I'm as baffled as you are as to why it won't go in.
Happy to refund and take it back if you believe it's at fault, but seems it works on other frames so perhaps the BB isn't faulty?

 

[markoc]

Can you try your BB in another frame to determine if the threads are damaged? That said, if they're damaged, it shouldn't be hard to see.

 

[Woz]

The UN51 uses steel cups. The DS cup is rather long and doesn't fully taper down. Compare the two cups and that of the UN71.

My guess is the threads are not deep enough in the BB shell, or clean enough somewhere around the DS chain stay. If you do some measurements it will help trouble shoot.

 

[ishaw]

@ishaw is this the BB I sold to you?
Apologies if it is!, I think we both agree it's in very good order - so I'm as baffled as you are as to why it won't go in.
Happy to refund and take it back if you believe it's at fault, but seems it works on other frames so perhaps the BB isn't faulty?

It is indeed the BB but it looks fine and I did try 8n another frame and it went in fine.

 

[ishaw]

Can you try your BB in another frame to determine if the threads are damaged? That said, if they're damaged, it shouldn't be hard to see.

It went in fine on a Kona frame, just doesn't like the trek, but every other BB I have tried (into double figures now) fits in the trek ok.

 

[ishaw]

The UN51 uses steel cups. The DS cup is rather long and doesn't fully taper down. Compare the two cups and that of the UN71.

My guess is the threads are not deep enough in the BB shell, or clean enough somewhere around the DS chain stay. If you do some measurements it will help trouble shoot.

View attachment 767659
View attachment 767660

It starts to get tight after a couple of turns

 

[Woz]

It starts to get tight after a couple of turns

It's just one of those combinations that doesn't work.

I've had a similar fitting problem with a Token in a alloy frame which had a small step inside the BB shell, even taking the cable guide screw out didn't give enough room.

 

[Woz]

The later UN52 is much more tapered and narrower around the center. The DS cup is actually part of the whole body and not removable.