French is larger, at 16 mm nominal, usually about 15.96 - 15.98 actual
English at 5/8" is 15.88 mm nominal, usually about 15.6 - 15.8 actual
I just measured a dozen cottered spindles, and
I think I found that I have only two French 16 mm diameter, the rest being English 5/8". Some of my Stronglight spindles, despite being made in France, measured in the English 5/8" range. Those 5/8" Stronglights are stamped with an A next to the overall length, and my one 16 mm Stronglight
doesn't have the A stamping, so I think the A might mean Anglais. I took pictures of all those spindles with their sizes written in sharpie,
stuck them here if anyone is interested.
The squirm from your French crank on an undersized English spindle can definitely cause problems with the cotter.
Care to clear up the apparent discrepancy between "
has never filed" and "
when he does file"?
Hmm, somehow I never once saw a mechanic use a grinding wheel on a cotter. Any wheel tends to make a hollow-ground shape, not a flat plane. I consider using a file a no-brainer since it's plenty fast and makes the desired shape "naturally".
Ha! If you define "ace" as someone who "could make cotters fit smooth", then we had more than a handful just at the shop I worked at in the early '70s. Because all of us could make cotters fit smooth (and work flawlessly even for strong riders). It was a large shop with a half-dozen mechanics, sometimes more. In order to not have to lay people off in the winter, they offered cut-rate complete overhauls, and we all did tons of them. Cottered bikes were still very much a thing then, so even new or 1-year-old bikes coming in for o'hauls had cotters, not all of course but probably a majority. (Not much Ashatabula/OPC IIRC, probably because those people didn't believe in paying a mechanic to service their cheap-ass bikes.)
You'd probably disagree that your definition of "ace" includes me and mechanics my age, but I would say the secrets are still very much with us. I was a teenager working after school and weekends at first, not one of the senior full-timers there until later, but I and other 'kids' there learned cotters well and quickly. By the time I was out of high school in '75, I felt I was an ace. I worked there full-time for a year as "assistant service manager" before going off to college, and did more cotters in that year than most currently active bike mechanics do in their entire career. I taught the tricks to new hires and supervised their work, but honestly just about any teenager can learn this quickly and not need any more coaching.
It's just not that difficult.
Mark B