Chainset won't budge - what I am doing wrong?!

[RecordAceFromNew]

Don't wait for the rainy evening....

Does the new BB look like the one below? The thread information is on there.
68 - width of the BB shell on the frame (measure your frame to be sure)
1.37 x 24 - That's british thread
If it says 36 x 24 then it's an Italian thread BB and won't fit your presumably british threaded frame.

Pete I don't think checking the label is going to help because the OP said the drive side fits. The issue is with the adjustable/left/non-drive side cup.

Checking threading direction also wouldn't help, because all left cups are right threaded. However an Italian adjustable cup would be too large and wouldn't even start. Try measuring with a vernier, or It might be possible to put both old and new cups adjacent on a flat surface and put a ruler on top of both to see if diameters are the same (else ruler won't be level).

 

[PpPete]

RAFN is right... as usual

 

[RecordAceFromNew]

RAFN is right... as usual

Unfortunately you are just being kind . I count myself lucky being right more than half the time...

 

[Barbelier]

Don't wait for the rainy evening....

Does the new BB look like the one below? The thread information is on there.
68 - width of the BB shell on the frame (measure your frame to be sure)
1.37 x 24 - That's british thread
If it says 36 x 24 then it's an Italian thread BB and won't fit your presumably british threaded frame.

Yeap that's the BB I bought and its British thread 68x110mm.
As suggested I'll check the old and new rings to make sure they're the same.

One thought - I greased the plastic retaining ring. Reading the instructions you're only supposed to grease steel and not plastic. But I can't imagine that's the reason it doesn't fit?

So other than buying another BB I'm a bit stuck now.
Can you buy just the retaining ring, maybe in steel, instead of the whole BB?

 

[PpPete]

Greasing the plastic ring won't make a difference.
I can only suggest that you look very carefully at the thread on the old cup, and compare it with the new one.
Maybe with a magnifying glass?
Is there perhaps some oddity or imperfection in the old cup that enables it to fit where the new one doesn't?

Like you I thought UN55 had a metal cup, not a plastic one.
It's pure guesswork... but one explanation might be that the one you have been sent had been returned to CRC* by some unscrupulous customer with a different cup - and CRC didn't notice and then sent it to you.

* or whoever you bought from ?

 

[Barbelier]

Greasing the plastic ring won't make a difference.
I can only suggest that you look very carefully at the thread on the old cup, and compare it with the new one.
Maybe with a magnifying glass?
Is there perhaps some oddity or imperfection in the old cup that enables it to fit where the new one doesn't?

Like you I thought UN55 had a metal cup, not a plastic one.
It's pure guesswork... but one explanation might be that the one you have been sent had been returned to CRC* by some unscrupulous customer with a different cup - and CRC didn't notice and then sent it to you.

* or whoever you bought from ?

Job done!

Carefully checked old and new rings and they are the same size and thread direction. However, the threads of the new retaining ring were full of a pale coloured thick gunk that I originally thought was some kind of coating or grease. Spent quite a bit of time carefully clearing out the threads so as not to damage them and hey presto it fitted.
Everything back together, gears running properly and my son is happy with his new "cool" chain set.

By the way the retaining ring was aluminium not plastic. My mistake and I was fooled by the lightest and the odd coating.

A good if sometimes frustrating learning experience. Will now be confident to sort out the bottom bracket on my son's MTB which definately needs replacing.

Thanks guys for all the helpful advice.

 

[PpPete]

Job done!

Carefully checked old and new rings and they are the same size and thread direction. However, the threads of the new retaining ring were full of a pale coloured thick gunk that I originally thought was some kind of coating or grease. Spent quite a bit of time carefully clearing out the threads so as not to damage them and hey presto it fitted.
Everything back together, gears running properly and my son is happy with his new "cool" chain set.

By the way the retaining ring was aluminium not plastic. My mistake and I was fooled by the lightest and the odd coating.

A good if sometimes frustrating learning experience. Will now be confident to sort out the bottom bracket on my son's MTB which definately needs replacing.

Thanks guys for all the helpful advice.

Probably a thread locking compound.