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I installed them with and 111 Centaur BB and it fits the same
That's already half the error you originally claimed, and just 1mm different from what I would predict if you asked me to guess at a starting point for substituting JIS for ISO.
If you look at all the potential sources of axial displacement error at the pedal end of the crank which are generated by the boss/spindle taper interface, you'll quickly realise that you can tell nothing about the "standards" from assembling one crank on two BBs, especially if you are just eye-balling it rather than taking proper measurements with the cranks in all 4 possible orientations on the spindle and with consistent bolt tension. How precise is your torque wrench?
If you wan't to reverse-engineer the standards, you should probably take at least 10 samples each from 10 manufacturers (5 JIS, 5 ISO) into whichever room your CMM is kept in, but my previous advice stands: check every individual installation for correct fit, because wear and tolerance stack happens. Either that or ditch the stupid things and get some cranks with a proper spindle attachment.
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gbj_tester
Eeyorecore
My Royal H has a modern TA crankset I ran it with a 107 Shimano BB for a few years, I tried it on another bike with much larger chainstays and the the ND side hit the chainstay. When I went to reinstall them on the Royal H I installed them with and 111 Centaur BB and it fits the same.
I also ran a GPM strada cranks on a Shimano BB for about 7 years.
So yeah, I’m not so convinced it’s a myth.