Bicycle Mechanics - BB Rethreading

I keeping thinking about older Raleighs, and their 26tpi BB being an issue when converting to modern cranks. Supposedly they can be retapped to 24tpi, but some worry about the threads being too weak. Why is that? I can understand that the threads may be weak; but is that for fear of the BB stripping out and ruining the frame; or because of fear of some sort of crash from the BB getting loose?

The problem is the Raleigh threads are exactly the same diameter as 24 tpi English threads so you will weaken the threads if you change the pitch.

A crash from a loose bb probably isn't an issue unless you ignore the symptoms for way too long. It isn't going to catastrophically pop out. Maybe retaping and using a lot of Loctite on the new bb is feasible.

One way to salvage a damaged English (or Raleigh) bottom bracket is to have it reamed and retapped with Italian threads and use spacers to widen the 68 mm shell to accept 70 mm Italian bb. The frame has to be valuable, at least to you, to bother with this.

I'm thinking of, get a cool old Raleigh and then upgrade with new bits as I feel like. Shedding weight would be nice, but I'm more concerned with the pains of initially tearing it apart to relube parts, then having to deal with cottered cranks--I'd rather tear it apart and rebuild "right" up front. I'm just guessing that a 30 year old BB ought to be torn apart right off for regreasing.

I'm guessing a bit of Loctite when I put in the BB would be a good idea--not the "never remove again" level but the version that just applies a bit more force than nothing. A good modern sealed BB shouldn't need touching for years.

Years ago, I replace the OEM Stronglight cottered crank in a friends '72 Super Course with a "modern" square taper Shimano crank and ran into the same threading problem. I used just the spindle from a proper length cup-and cone bb and retained the original Raleigh cups which were in good condition. You might try this first and rethread the frame only if the stock cups are in bad shape.

That is true too. I am looking at a Twenty for fun, as I spend one weekend/month away from home. The Twenty supposedly needs a really long spindle though, but it is another option. I don't have a junkbox of parts (yet).