Bottom Bracket woes

[yostumpy]

A chum has a trek road bike that had external bb. He has bought a new chainset with shorter cranks, and a square taper bb. He's tried to fit it, but the non drive side cup maxes out on thread, 1/4 " from the bb shell? What to do? Cut down lhs cup, and risk ruining thread? Get BB shell recut with longer thread( aluminium frame)? Or use the plastic spacers that came off the external bb? He needs help!

[NickJP]

Take the frame to a decent bike shop that has a set of bottom bracket taps, and get them to chase and face the BB shell. Take the replacement BB as well, so they know how much additional thread is needed.

[Valbrona]

In the case of some square taper bottom brackets the LH isn't meant to screw all the way in and correct fitting leaves threads on the cup exposed. Read instructions.

[yostumpy]

In the case of some square taper bottom brackets the LH isn't meant to screw all the way in and correct fitting leaves threads on the cup exposed. Read instructions.

Ah, didn't come with any instructions. Never heard of this before. It's a 'First Components' one from Spa cycles.

[iandusud]

I would try running the l/h cup in on its own and see if it goes in all the way. If not then measure the gap and then fit the BB as per normal, i.e. r/h cup first, then l/h cup and remeasure the gap. As long the gap is greater than before you can be sure it is OK.

Ian

[cycleruk]

Which BB has he got?
Any chance he has a 73mm BB (MTB) instead of a 68mm (road).
Try the cup on it's own and if still no luck than get LBS to chase out the threads, as mentioned above.

[yostumpy]

Ok folks, problem resolved. He lives across the road, so I popped over this morning, with a new Shimano UN 55.
Basically the LHS cup wouldn't screw in all the way.( First Components) , so I took out the LHS and the thread was a bit 'bitty' So I cleaned the threads, put on some thin lube, and tried again, and it would only go in 1/2 way, and was very stiff, but not crossed.
Ok, so I took it out again, and tried the old ext bb and it went in nicely, took it out again. Then cleaned threads inside again, spot more lube, tried UN55 cup,a little stiff by hand, but ok, worked back and forward ok, did up nicely with tool. So remove again and tried FC bb cup, no go! So I have given him my UN55, as it all works nicely, and the other new BB will be going back.

Thanks for all the help.