I have a square taper bottom bracket. How can I tell whether it is a J.I.S. or ISO taper with a minimum amount of disassembly?
FWIW, it's a Ritchey from 2004. However, it would be nice to get answers that apply to other brands as well.
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The easiest method is measuring the outmost part of the spindle, like the answer of Pedalphile suggest, however there seem to be differing measurements.
These are the values I found(source in german):
ISO 1991: 12,50 mm
JIS: 12,63 mm
ISO 2015: 12,73 mm
I was able to to reproduce the ISO 1991 value by measuring my own bike.
This is a minor answer expanding on a comment I made to the original post. If the crank in question was Shimano, then the BB has to be a JIS taper, unless someone didn't know that Shimano square taper cranks have always used the JIS taper. Similarly, a Campagnolo square taper crank would always use ISO taper.
Unfortunately, with other crank manufacturers, the picture may be less clear. For example, as ojs pointed out in comments, TA and Stronglight (two French manufacturers) probably used ISO for a long time, but I can confirm that current TA square taper cranks are designed for the JIS taper, because I have one. ojs points out that older European brands may be ISO.
The bike model and year may not offer us much information if we know the crankset and BB in question. A 2004 Ritchey road bike could conceivably have been built up with Shimano or Campagnolo cranks, or maybe even some obscure third party if the owner wanted that. If it were a mountain bike, then I'm less familiar, but I assume that would shift the potential set of components away from the smaller European brands, so I would assume that the JIS taper is more likely.
You could use a known JIS/ISO to compare to.
Other sources confirm they are the same angle though. Minimum disassembly will be taking off one crank.
I can't find anything that states exactly what size the taper goes down to.