I came to the realization today that I don't know anything about how
threaded nuts are sized. Up until now, I'd just blithely assumed a 1/2" nut
meant it took a wrench of 1/2" jaw span to fit it.
Imagine my surprise when I found out the flat side span of a nut isn't the
size. So, what how is size determined? Is it measuring across The nut
Definitely not! (Unless I misunderstood your wording.)
Some people might call for a nut by the size thread it's going to screw
onto, without stating the thread pitch, so they'd say, "Get me a half
inch nut, and be quick about it.", when what they really meant was a 1/2
x 13 TPI nut, probably the most common thread pitch for construction
The span across the nut for a given thread size will vary depending on
what the nut is going to be used for, but if you find a nut which
measures 1/2" across the flats, be it a hex or square nut, then a 1/2"
wrench is what fits it. (Duh...)
A nut size is determined by the size of the threaded hole. Thus, a
1/4-20 nut has a nominal 1/4" hole with 20 threads per inch. Metric
nuts, same story, but pitch/size is expressed differently.
The wrench and/or socket you use bears no clear relationship,
obviously. The size of the wrench you use is whatever fits snugly, and
is expressed as the size across the flats.
I think the Witworth (semi-obsolete UK system) measures across the
points. I remember on an old Austin-Healy (gee, how I miss that car!)
that I had problems with my US wrenches fitting properly.
For US nuts/bolts, use fractional inch wrenches and/or sockets. For
metric ones, use Metric wrenches/sockets. Some sizes are pretty close.
I notice that the last set of 3/8" drive sockets I picked up had both
fractional inch and metric sockets in the set.
You need both, as I've found that my 2000 Jeep Cherokee has both
metric and US fittings!
For metric, the *thread* size is usually expressed with a preceding
capital M and the *hex* size has a lowercase mm appended. For example,
most M8 thread bolts/nuts use a 10mm wrench. An M10 thread bolt/nut
will often use a 13mm wrench. There are exceptions of course,
sometimes auto makers use a massive M12 bolt and put little tiny 10mm
head on it (I'm glaring at you, Chevy). Hardware store bolts will
usually not have such oddities, though.
Usually when you say "I want a 13mm bolt" you mean you want an M10
thread bolt with 13mm flats. Or you asked for an M12 bolt, you'd
probably get an M12 thread bolt with 17mm flats.
Just may be by the end of this century every thing will become
standard again unless some smart ass bolt manufacture decides to maybe
make every thing a left handed thread. The world seems to be moving in
the wrong direction, so just, maybe its is time to reverse things and
go in the opposite.
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