Bicycle Mechanics - Removed a damaged BB - possible damage to threads in shell (pics)
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Hello. The fixed cup of my BB was not sitting flush with the frame and had the appearence of being cross-threaded (though it wasn't, since it hadn't been removed and re-installed for years and wasn't always sitting wrong like that).
I finally got it removed by welding some nails to it and using them for leverage, plus plenty of WD40 once I got a bit of movement:
Upon inspection, it is pretty clear what had happened to the fixed cup. Look at the first pass of the thread from the outer edge of the cup:
The threads in the BB shell, on the frame, don't look nearly as bad, but I wonder if there might be any damage?
What do you reckon? Might I run into trouble screwing in a new BB, and/or risk it not sitting in securely enough?
If you don't file out the damage you might have problems starting a new cup without cross threading. You have a few ways to approach it.
You can grind a few notches across the first few threads of the cup you just removed, and try to start it and see if you chan simply chase out the dings. Be sure to oil it well with a heavy cutting oil, though any heavy oil will pass, and be very careful about cross threading.
You can try to use the point of a triangular file run along the thread to remove or bend the dings.
You can use a half round file and filing at an angle simply remove the damage down to the root or at least to good thread. This is the route I'd take. The loss of a bit of thread area on the outermost thread or two won't in anyway compromise the thread, and there's little risk of compounding the problem by cross threading.
BTW- that was a creative way to remove a frozen cup. I've done similar by welding a bolt to the face and using a wrench. I see where the cup nearly fractured, though. Next time try to use an electric welder and either work faster, or in sections resting and cooling the cup in between so the heat won't carry in so deep. As it is the good threads on the cup basically chased out the thread on removal so the damage was minimal. Good job.
07-20-10, 09:37 AM
Take the frame to your LBS and have them "chase 'n' face" the shell. That way you'll know it's been done right.
07-20-10, 10:19 AM
+1 I had this problem with a Schwinn World Tourer, and when the LBS was done, it was as good as new.
The LBS is always an option, but given that the OP has done the hard part already, there's no reason to pay an LBS to finish the easy part.
The BB shell was already faced, and if the OP raised some metal, it can safely and easily be filed flat. Only a high spots create a facing issue and a local low spot or two is meaningless. Otherwise the threads are in excellent shape, and need only minor dressing. The OP's welding isn't a work of art, but I assume a bit of simple file work is well within his skill set.
The OP's welding isn't a work of art, but I assume a bit of simple file work is well within his skill set.
Well, we'll see - haha. By the way that was an electric welder I used. I'm not very good at welding but also the cup was apparently made of a MUCH harder steel than the nails, so by the time the cup was getting hot the nail had dripped away onto the floor. Joining mismatched metal like that is always tricky.
So basically I'll just try to poke the threads back into shape a bit, one way or another. I'll see how easily the old cup will screw down a few turns, with plenty of oil. If it goes in nicely then maybe the thread will be ok. Otherwise I'll try dragging along the thread channels, and finally - because I'd have to buy a file for it - I'll file off the top few loops of thread altogether.
LBS will be a last resort, I don't have much money.
07-20-10, 10:58 AM
One advantage of having the LBS 'chase' the threads (I agree OP probably does not need to have it faced) is that a shop-quality thread chase setup will insure that the cups are properly aligned when completed. I think it is well worth the $10 I spent for mine to not have any worries about BB tightening/loosening issues.
07-20-10, 11:03 AM
Yea, you can clean up the threads by running the tap thru it, that is chasing threads rather than cutting them ,
bike BB taps have a centering guide so both turn around the same axis,
but the shop will charge you a service fee.
Velo Orange sells a very nice square taper BB that does not fix to the frame with the threads , it self tightens with in itself,. with the lock rings [need proper tool] on the outside ..
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