Joined: Nov 2006
Posts: 19,811
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From: NJ, NYC, LI
Bikes: 1940s Fothergill, 1959 Allegro Special, 1963? Claud Butler Olympic Sprint, Lambert 'Clubman', 1974 Fuji "the Ace", 1976 Holdsworth 650b conversion rando bike, 1983 Trek 720 tourer, 1984 Counterpoint Opus II, 1993 Basso Gap, 2010 Downtube 8h, and...
I too have been thinking about tapping a 26 tpi Raleigh BB to 24 tpi BSA threads, and I'm not sure I like the idea at all. Some members have posted remarks such as "the difference between 26 tpi and 24 tpi is only two per inch, so it's no big deal..." or some such thing that doen't make sense to me, so I made the graphics below.
The first graphic represents a 1/2" section of metal with 13 threads cut into it, followed by the same section of metal with 12 threads cut into it, and finally what's left if you first cut 13 and then 12. This represents 1/2" of thread. So in an inch of thread, you are going to have two strong threads, four to eight weaker ones, and a bunch of very weak ones. Since ALL the new valleys will be a little thicker than the old ones, ALL the remaining ridges will be thinner than the old ones.
The second graphic shows what happens when you thread your new 24 tpi cup into the BB, and thread your new 24 tpi lockring onto the cup, and tighten the cup against the re-faced BB shell.
In this graphic, anywhere you see white between the threads, the threads are not in contact.
When you tighten the lockring, it pulls the cup to the left, so the left sides of the cup ridges press on the right sides of the BB shell ridges. At first, the load will be held by the right sides of about six ridges; as you put more pressure between the lockring and the cup, the steel of the BB shell may deflect a little, gradually spreading the load to other ridges. But you risk shearing all the ridges off. Sounds a bit risky to me.
These are not engineer-grade drawings, obviously; they are intended only to illustrate my thinking about the problem. Needless to say, if there are flaws in my thinking, I hope someone will point them out to me.
Would it make more sense to leave the BB threads alone, and grind off all but maybe 1/4" of the threads on the cup? That way, if it fails, you only ruin a cup and a small portion of your BB.
Alternately, could you melt some brass into the threads of the BB before you cut the new thread into it? I'm sure it's not a perfect solution, but it must be better than nothing.