David, French thread was 14 mm x 1.25 mm, English is 9/16 inch x 20 TPI. That means English pedals had a thread diameter of 14.28 mm versus 14.0 mm for French. These are nominal. If you followed the thread on TA pedals a couple of week ago, you know the actual diameters were usually 0.1 mm to .02 mm smaller. 20 TPI works out to a 1.27 mm pitch, so the pitch was nearly identical. A lot of old French cranks are marked 14 x 125 on the back of the arms.
What you observe is typical of English pedals in French cranks, i.e. they go in OK for a couple of turns, then tighten up. You can buy taps for tapping out French threaded cranks to English thread. Several shop owners on the list sell these. Since the difference is only 0.28 mm, tapping is fairly easy and was commonly done back when French cranks were more common. Truth is, you can "tap" French cranks just by forcing the English pedals in, but you risk damaging the threads that way. Of course you can still find old French thread pedals if you look. Sheldon sells some, and BC just posted some NOS French thread Zeus pedals for sale.
Regards,
Jerry Moos
Houston, TX
Hi gang,
I have been sorting cranks and find I have a number of crank arms with perfect threads but none of my English threaded pedals fit, but the apparent difference seems very slight ie the overall hole size would seem the same, but the pedal ends will catch the crank thread then go no further. I assume that the cranks in question are French threaded and would guess that the difference is possibly just the tpi in the same way that Raleigh bottom brackets differ from other Brit bbs, but can anyone suggest a rule of thumb on the different sizes as typically all my 50s/60s books are way too vague! Many thanks.
David Blight
Exeter UK where spring is in full swing.