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I'm fixing up an old bike I bought in Belgium. Not sure about the exact brand/year. The bottom bracket axle is branded "Flandria". It's a cottered axle. I want to replace this with a square taper bottom bracket. Details of my current bottom bracket:

  • frame is 70mm wide (where to bottom bracket goes)
  • threads are 24tpi, both sides screw in clockwise
  • bottom bracket diameter is 33.9mm

I tried fitting a British style bottom bracket first (excuse my incompetence :) ). That didn't screw in because of the thread direction of course. The right threaded end does screw in easily, therefore my conclusion that it's 24tpi.

After that, I tried an Italian style one. The diameter of this is too large, so doesn't fit into the frame.

It can't be a French style bottom bracket either, as the frame is 70mm wide. My information source: https://www.sheldonbrown.com/cribsheet-bottombrackets.html

Any ideas?

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  • Interesting! so are you saying the British 'right-threaded' cup will screw in easily both sides? does it bind up at any point, or go all the way in?
    – Swifty
    Jul 22 '19 at 16:12
  • Ideas? How's this: Belgium is almost France, so you have an almost-French bottom bracket. On a serious note, 1 mm threads are close enough to 24 tpi that a 24 tpi component that's short enough might work even if the threads in the shell are actually 1 mm. Shimano BB-UN55s are made for 70 mm M36 bottom bracket shells. Jul 22 '19 at 16:14
  • @Swifty: The british right threaded cup goes all the way in without binding up at any point, indeed.
    – jvermeulen
    Jul 22 '19 at 16:27
  • @AndrewHenle Makes a lot of sense. I can only seem to find Italian style BB-UN55s that are 70mm wide though. The diameter of these is too wide for my frame. Can you point me to a specific BB-UN55 model?
    – jvermeulen
    Jul 23 '19 at 9:11
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While the exact specification might be mysterious, why don't you use the existing cups with a new (or used) square taper spindle? Hopefully they have been looked after well!

You should be able to find one with suitable dimensions fairly easily. I can identify ebay listings with spindles in many lengths for exactly the conversion you mention.

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I think it's probably French. If it's not then it's something brand-specific, and JoeK's solution becomes likely the only option.

If you don't have a thread pitch gauge, counting twenty threads and corroborating that the 0 and 20 line up exactly with 0 and 2cm on a ruler is another way. Then carefully lay the ruler across the center of the cup and corroborate 35mm.

Exact shell width mattered somewhat less in the days of cup and and cone. One of the many nice things about just cutting to the chase and solving this kind of situation with Phil cups is they're non-shouldered so you can dial the Q even with an oddbal shell width.

If it really is something weird, another plan is always just have someone with a lathe make you some Phil cups.

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