Take the 2-minute tour ×
Bicycles Stack Exchange is a question and answer site for people who build and repair bicycles, people who train cycling, or commute on bicycles. It's 100% free, no registration required.

I installed a new sealed bearing in my bottom bracket. Unfortunately after tightening it most of the way, it jammed and I realised I'd cross threaded it.

I then removed it and reinstalled it. It tightened up no problems and seems secure now. I went for a short ride and it appears to be working fine.

I am going to use this bike as a fixie build and am planning to put a fair amount of effort and expense into the frame, so I don't want to continue with this bike if I've damaged the bottom bracket significantly. This link indicates that I may have done irrepairable damage.

So my questions:

  • Given that it now appears to be fine, how can I tell if I have damaged the bottom bracket?
  • If it is damaged, are there methods to repair it, or should I discard the bike and get another donor bike? I don't want to put a lot of work into this frame and then in 12 months discover that it's not usable long term
share|improve this question
1  
Did you examine the threads? How badly mangled were they? How mangled were the cups? How hard was it to install the cups correctly? You kind of have to use your own judgment, but in practice very little stress is placed on the cups (especially if a cartridge is used), and one can get a bit of "insurance" by using some thread lock compound on the cups. –  Daniel R Hicks Dec 1 '11 at 1:07
    
@DanielRHicks The thread looked fine to a casual inspection. Installing the cups correctly was easy, there was very little resistance. –  Mac Dec 1 '11 at 1:20
    
Did you torque them to spec? –  Daniel R Hicks Dec 1 '11 at 2:32
    
@DanielRHicks no... I don't have a torque wrench. I plan to take it to a bike shop and ask them to torque it before I finish it off and ride regularly –  Mac Dec 1 '11 at 20:08
    
Some here get their shorts in a knot when I suggest this, but if you can estimate, say, 10 or 15 pounds pressure with your hand (try lifting an object of the appropriate weight) you can come halfway close by figuring out how many inch pounds it is, measuring the length of your wrench, and dividing the inch pounds by the length of the wrench -- that's the amount of force to put on the wrench handle. Of course, since you're estimating you should undershoot by 30-50% (or however much you trust your judgment), but you still get in the right ballpark. –  Daniel R Hicks Dec 1 '11 at 22:11

4 Answers 4

up vote 7 down vote accepted

You probably didn't remove material when you cross-threaded the bottom bracket shell in. If it's a steel frame, you can cut new (correct) threads with the correct taps. The taps are quite expensive, (they have to line up, and one is upside down, find a shop or local framebuilder who has one.

Baring that you should chase the threads : grab old steel bb shells, cut two triangular grooves in them. You want a perpendicular cut on the trailing side of the thread. (Ask the internet for images of a thread chaser). Fill the grooves with thick grease, thread them in and out wipe it down and you're done.

You can wrap some teflon tape around the threads to make them tighter as well.

Either way you probably shouldn't "invest" money on this frame anymore, maintain it and once you've ridden it through, turn it into a shop stool.

share|improve this answer
1  
+1 Love the home-made thread chaser idea! –  cmannett85 Dec 1 '11 at 20:52

If you were able to get it screwed in correctly, and nothing feels loose, then it can't be damaged that bad. I wouldn't worry about it.

share|improve this answer

Firstly clean all the metal bits floating about the BB from the threads being cut. You can usually just look at the state of the threads at this point to see how bad the damage is. Your eyes should be able to follow the grooves around uninterrupted, if you can wind the BB back in OK, and it tightens as normal then you should be fine.

Because the force a BB is subjected to is perpendicular to the thread direction, the chance of it pulling out is very slim.

If you ever do truly annihilate your BB threads (remember cross threading is a pretty common issue in engineering), you could probably get an engineering works to drill out the existing thread, press a steel shim into the BB tube, and then thread that. I've seen it done on smaller fittings, so I don't why it wouldn't apply to larger ones.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.