The different hex sizes on fasteners 8mm thread size and larger are sometimes a problem, sometimes not. And sometimes the flange head found on many JIS fasteners is essential to the application (and sometimes not).
For example, my Japanese motorcycles are littered with M6, M8, and M10 fasteners with flange heads on the bolts and 10, 12, and 14mm hexes. The most commonly available hardware has no flange and 10, 13, and 17mm hexes. Sometimes this matters quite a bit. In any case, it's very damn annoying to run across an M8 fastener that has been replaced and have to go back and grab a 13mm socket instead of the expected 12.
And JIS thread pitches 10mm and above are NOT the same as the others (or maybe you have enough ugga-duggas to not care...?). For example, the usual thread pitches for M10 and M12 Usian and Yurpian stuff are M10x1.5 and M12x1.75 (and this is the majority of hardware you'll find in hardware stores), and in JIS it's M10x1.25 and M12x1.25.
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