I'm swapping out the bottom bracket on my Bertin C37 and was planning on using the IRD French cups with either a Tange or IRD BB, but I just measured the shell width with my digital caliper and its right around 70.80mm?! I was under the assumption that French bottom brackets were 68mm. The thread on the cup I've removed measures 34.75mm... I haven't attempted to take out the fixed cup yet but am now unsure of what I'm dealing with. The BB that was in there originally is a TA but the spindle is pretty beat up, cups look okay. I ride in the rain a lot and wanted to try a sealed cartridge. I think the bike is maybe late 60's or early 70's? Any insight would be greatly appreciated!

Shell width is only very loosely correlated with thread spec. While 68mm is the most frequent width for French-thread shells, anything is possible.

You say the original bottom bracket was TA. TA and Stronglight differed from most other bottom brackets in using the same axle for both 68 and 70mm wide shells, and varying the thickness of the cups to make up the difference. If your Bertin is set up to use a double crank, that would mean a#344 spindle.

TA cups use rings to indicate thread spec:



And TA lockrings use knurling to indicate thread spec:

Wow. That is way out of spec. I think what has been demonstrated here is that old system bottom brackets were remarkably adaptable. And your spindle survived from late 60s to 2018. With a track record like that replace with a new one just like the old one. They are not that hard to find and will be a lot cheaper than anything new. TA was finished a bit better than other bearings from Verot et Perrin but you are allowed to swap in a Stronglight or Nervar.

-----

Verktyg hath writ that following the fire at the factory there was a period where Bertin cycles were constructed in Belgium. If your shell is ~70mm that would tend to indicate a BSC threading. This combination was widely used in Belgium. Suggest you verify threading is metric prior to attempting to fit a new bottom bracket.


​​​​​​​-----

I thought only Italian used 70mm bottom brackets. That said, I have one bike with Italian threads and a 68mm wide shell, so as has been pointed out the design is flexible. Based on the OP's cup measurement, I'd guess French threading, but it is definitely worth verifying that before trying to thread something different in there.

The nice thing about the IRD bottom bracket design is that the cups can be changed independent of the bearings and spindle (it's not really a cartridge but it does use sealed bearings).

------

70mm BSC is quite common in Belgium. Most notably from Flandria.

Then there are those gazillions Raleigh products with their 71mm shells.

Japan produced millions of cycles with 71mm BSC shells.

---

Bertin factory fire was in 1972.

​​​​​​​-----

Wow! Thank you to everyone for all of the information, it was far more than I expected. I screwed up! I mismeasured the shell forgetting that the fixed cup was still in there... duh. It was way too late and dark and I didnt see it. I was running errands today when I thought of it. Oops! Measured in daylight it IS in fact 68mm after all. The cups have one ring and the knurling is criss crossed so it should also be french threaded. Sorry for the false alarm and thanks again to everybody for the help!

One other question: has anyone used the IRD cups with a Tange 3922? I know that Tange makes the IRD bottom brackets and it looks like they have the same style removable cups and bearings with the Tange using 37/17/10 bearings and the IRD using 37/16/10 from what I've found online. It should work right? IRD's site says they'll work with some Tange BBs but their official store says for IRD brackets only.

I've seen more than a few Japanese bike-boom era frames with 70mm shells and English thread.

Italian track frames used to have 65mm shells, and bikes with stripped English or metric (French or Swiss) thread shells can be re-tapped to use Italian thread.

So, you should not really have any confidence that shell width will tell you anything about the shell thread spec.