makes sense to me. The wrench size is the distance across the flats,
the nominal size of the bolt is related to the size of the threads.
SAE works the exact same way. e.g. a 3/8-16 UNC bolt or cap screw
typically requires a 9/16" wrench to tighten.
The only exception that I'm aware of is Whitworth, although I'm sure
that someone will correct me with an even more obscure example.
And to think that there was consideration of official adoption of the
metric system back when Thomas Jefferson was President. How much simpler
things would be if the chang was done then, before industrialization. The
country was still in a rebellious mood when a national currency based on
the decimal system was adopted, but by Jefferson's time had already
turned conservative enough that a change to metric was defeated.
There is always an easy solution to every human problem -- neat,
plausible, and wrong." (H L Mencken)
The metric system, once acclimated to, is so much simpler and straight
forward that it isn't even funny. It makes it VERY easy to figure things
in your head.
My only trivial complaint is that the Celsius degree is a tad large for
some things... but I can easily deal with that.
Same reason that if you have a double-open-end wrench set, the wrench
with 5/8" on one end will likely not have 11/16" on the other - so
that for most common combinations of bolt and nut you will be able to
tighten the assembly with only one wrench set.
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