Bottom bracket fitting problems, help needed

[ishaw]

Usually a simple task right? Not this time for some reason.

Frame had a BB fitted,came out easily, which was a bonus as it's got try bearings.

I bought a very tidy un51 unit in the correct size for frame and intended cranks but from here the problems began.

Nds cup screws in fine, no drama, but the drive side does not want to go in very far at all before it starts binding. I didn't want to damage it or the frame, so backed it out, made sure it was seated correctly and tried again, same result.

I've refitted the old BB, no problems, tried another, no problems, even tried sn ht2 cup which also went in with no problems.

I've tried the offending BB in another frame (Kona alloy) and it went in fine, as did the others, so why am I having such problems fitting the BB on the frame I need it to fit in (trek 930 steel)?

 

[synchronicity]

Because the threads aren't cut deeply enough.

 

[ishaw]

Do you mean on the frame? Strange all other bbs I've tried screw in as expected.

Any ideas on how to solve the problem?

 

[synchronicity]

Since you state that the same BB cups screw in fine on the other frame, I'd wager it's mostly the Trek's fault (but not entirely, otherwise all BB's would fit, but this one doesn't).
It's a tolerance issue when you try to engage that particular combination. I.e both sets of threads might not be cut deep enough.

 

[synchronicity]

You'd either need to do one or more of the following:
1) Use grease (you should be using it anyway)
2) Use more force (and risk wrecking the cups, especially if they are alloy)
3) Recut/retap the threads (easier to recut female threads in the frame probably)

 

[ishaw]

Thanks.

1. Already using grease (copper)
2. Have put in a fair bit of force, fearful of damaging things. BB got quite warm doing this.
3. Don't have the tools for this. I'll try to find a donor BB that may have the same issue but I've tried a few and all fit nicely aside from the un51.

 

[synchronicity]

Ah, copper is more for titanium on titanium bits, anti seize wrt corrosion/galling.
That's all I use it for. It doesn't really help to reduce friction, probably hinders it because of the small particles.

Are the female threads a bit rusty? I'd clean them out with a wire brush and use actual grease (for what it's worth, that's my advice).

 

[synchronicity]

https://www.webbline.co.uk/shop/bottom-bracket-taps-bsa-pair/https://www.merlincycles.com/cyclus-bsa-bottom-bracket-tap-230298.html
What is it about £50 for a set.
Up to you whether it's worth it.
May be cheaper to visit the lbs for this one.
Or simply use a different BB.

 

[synchronicity]

The other risk being that if you do manage to get it in now (with some effort), imagine how hard the bastard will be to remove in 10 or 15 years' time?

 

[ishaw]

So I have tried 10 other bbs tonight, all screw in nicely. They were of varying quality ranging from cheapo Shimano through to top end, even tried an Isis BB. Cups were a mix of steel and alloy. Tried the BB I want to fit after all the others and still it won't go. It screws in a little way, gets tougher and by about the time half the thread is in, it's so tough I can't make it turn any further without using more force then is really safe or sensible.

Fyi I've used copper grease for years, it's ideal for the purpose. For ti parts a more specialised grease is needed, I use ti-prep iirc.

I can't justify spending £50 on taps for a single use, especially as all other bbs fit. I may just have to find another BB in the correct size and see if that screws in.

All very odd.