# how nut sizes are detemined

• posted on May 4, 2007, 10:32 pm

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 from point-to-point?
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on May 4, 2007, 10:44 pm
rb wrote:

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 type stuff.
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...)
HTH,
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on May 4, 2007, 10:45 pm
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!
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on May 5, 2007, 1:24 am
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.
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on May 5, 2007, 5:18 pm
On 4 May 2007 15:45:12 -0700, professorpaul wrote:

Having owned a Singer Nine, a Morris Minor, and two MGs (A & B), I once bought a set of Whitworth size wrenches from Sears.
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on May 5, 2007, 12:38 am
It's usually hereditary, through the genes.
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on May 5, 2007, 1:27 pm

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.