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?

  • 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 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. – Andrew Henle Jul 22 at 16:14
  • @Swifty: The british right threaded cup goes all the way in without binding up at any point, indeed. – jvermeulen Jul 22 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 at 9:11

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