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madmike
03-29-2009, 06:58 PM
So I'm building a streetfox, and as I'm messing around with the bottom bracket
shell I manage to cross thread one of the cups, or try to put the wrong one
in the wrong side, or something. Any how the threads are gouged up to the
point where the cup absolutely won't go in straight no matter what. After an hour or so of trying to repair the threads by poking at them with little tiny files
and a carbide scribe I remembered an article I read about using a tap in a lathe to cut threads on an internal diameter. You need a tap with the same thread pitch as your desired I.D. threads, which in this case was 24 threads per inch,
so I grabbed a 3/8x24 tap and a tap handle and threw the b.b. shell in a bench vise. With the tap rotated so just the corners of the cutting edges engage in
the existing threads, and held carefully parallel to the shell, it was only the work of a minute or two to restore the damaged threads to as-new condition.
It also works like a charm to remove rust and greasy mung from the threads
on the cups.

So make sure you never throw away those old issues of Home Shop Machinist
or Machinist's Workshop.

pcorbett
03-29-2009, 07:21 PM
Excellent. You have just saved me of massive bouts of swearing:furious:. Neighbors are much happier now. Always a bummer to deal with those bunged threads. Thanks for the tip.

Welcome to AZ,

Pete

TheKid
03-29-2009, 07:39 PM
If the threads are beyond repair, you could use a threadless BB. They also work with 1 1/4" conduit, if you need a BB shell and can't find one.

http://www.biketoolsetc.com/index.cgi?id=68269755716&d=single&c=Components&sc=Bottom-Brackets&tc=Cartridge/Taper-Spindle&item_id=YS-BB99368113

greenevegiebeast
03-29-2009, 11:32 PM
go by your local auto parts store and see if they have a "Loan a tool" program if they do they might have a thred repair kit in it.

the thrid repair kit will not have a tap or die that sixe , but. what it might have are handy little tools called thred files. with their and a little elbo grease you could clean up tthose threds if they are not to bad.

John Lewis
03-30-2009, 12:27 AM
Good tip that to keep in mind.

What I did was to make a set of thread repair "taps" by taking a pair of BB bearing cups and cutting a series of lengthwise grooves in them across the threads with a thin dremel disk. They will clean up a bad thread quite well. If all else fails I can get the LBS to use their BB chaser taps on it.

John Lewis

TheKid
03-30-2009, 03:03 AM
This thread has some good threading tips. I used BB cups for repairing crossed threads in a disc brake adaper. Because they're so hard, they work well. But I didn't think of cutting grooves to clear the filings. It would have made things easier for sure.
Another thing I found was that a cromoly BB shell will cut threads on some alloy hubs. I didn't think of it until now, but if you use the left threads, then make a disk adapter to weld onto a 3/8-1/2"" wide piece of the shell, it will solve the problem of the left disc adapter unscrewing when used on the left wheel of a tadpole or quad.

Butterbean
04-04-2009, 02:11 PM
I'm sure that worked on the left side of the BB, but how did you chase the threads on the right side, since that side is usually reverse threaded?

TheKid
04-04-2009, 05:06 PM
I'm sure that worked on the left side of the BB, but how did you chase the threads on the right side, since that side is usually reverse threaded?

Use a left side BB cup, or buy a left thread tap.

http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=1736760&PMT4NO=0

Butterbean
04-05-2009, 09:31 AM
I've never seen a reverse thread tap with 1.370" x 24 TPI, except as part of a BB tap set...and they start at $200. The tap you linked above is only a 3/8" tap, which is way too small.

25hz
04-06-2009, 11:13 AM
I'm sure that worked on the left side of the BB, but how did you chase the threads on the right side, since that side is usually reverse threaded?

Just use any tap with the same pitch to clean the L/H threads too. A smaller tap (like the 3/8" one mentioned) is just a scraper anyway. It works fine on cleaning up the threads, left OR right hand.

TheKid
04-06-2009, 02:32 PM
You need a tap with the same thread pitch as your desired I.D. threads, which in this case was 24 threads per inch,
so I grabbed a 3/8x24 tap and a tap handle and threw the b.b. shell in a bench vise. With the tap rotated so just the corners of the cutting edges engage in
the existing threads, and held carefully parallel to the shell, it was only the work of a minute or two to restore the damaged threads to as-new condition.


In order to get the most out of these forums, read the hints carefully. Sometimes it's easy to miss something important. In my haste, I have sometimes missed something pertinent, as I'm sure others have done as well. Just thought this would be a good time to point that out.