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Bolt Hex vs Wrench Size

Bolt Hex vs Wrench Size

When a bolt head hex is designed for a 3/8" wrench - which is supposed to be 3/8" (the bolt hex or the wrench) and what is the standard clearance/tolerance?

RE: Bolt Hex vs Wrench Size

The wrench is oversize relative to the nominal dimension, and the hex (bolt head) tolerance ranges from "right on" nominal to slightly undersized.  For the 3/8" hex example, the wrench opening (flat-to-flat dimension) is 0.378 to 0.384.  The bolt hex is .375 to .362.  Both are standard for unified standard bolts & nuts per ANSI B18.2.

A good source for all of that kind of information, and much cheaper than buying all the relevant spec's., is any copy you can find of the Machinery's Handbook (esp. if you buy an older, used copy).  I started my career with a borrowed copy dated ca. 1930's, now own my wife's old copy (19th ed., 1970), and we also have a couple of more modern versions lying around.  Surprisingly :) there hasn't been much change to standard bolt hex and wrench tolerances over the years... can you imagine the outcry if millions of backyard mechanics had to throw out their trusty Snap-On's every 10 years or so?

RE: Bolt Hex vs Wrench Size

btrueblood is right on.  The specs for inch wreches are right in Machinery's Handbook.  I've looked for them before and must have missed them.

Anybody have an idea if metric wrenches are similar in their relationship to the bolt hex?

RE: Bolt Hex vs Wrench Size

btrueblood, thanks for the helpful post - exactly what I was looking for.

RE: Bolt Hex vs Wrench Size

The old standard for hex head sizes is bolt diameter + 1/2 the bolt diameter = hex size.

Example 3/8" bolt head size is 3/8" + 3/16" = 9/16" hex size.
The old standard for nuts was to add 1/8" making the nut hex 3/8" + 3/16" + 1/8" =11/16" , but now the nut and bolt hex are the same except in the heavy hex nut series.

Never toss old machinery's handbooks, some of the newer ones edit out specifications they consider obsolete, but are still very much alive in the older machinery.

RE: Bolt Hex vs Wrench Size

BobC is correct.

The hex size is 1 1/2 times the threaded shank of the bolt in question.  So your 3/8 NC bolt will have a hex size of 1.5(0.375 in)= 0.562 in = 9/16".  Incidently the height of the hex is exactly equal to the threaded diameter or 3/8".

This is obvious from the Machinist Handbook, to which I concur, is perhaps the best readily available source.

Kenneth J Hueston, PEng
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

RE: Bolt Hex vs Wrench Size

I am always impressed with the expertise people like to share on this board regardless of how relevant it is to the original topic (just kidding guys).  Thanks for the useful information - I'm adding the Machinery's Handbook to my must have's.

RE: Bolt Hex vs Wrench Size

I thought we had the ANSI spec for metric fasteners around, but after looking again yesterday, I couldn't find it (also am now missing my copy of ANSI Y14.5...must be gremlins!).  Anyway, I would suspect that wrench opening tolerances would be given within the ANSI spec for hex-head metric fasteners.  Could be wrong, though - and this time the Mach. Hdbk. fails to clarify.  If I had to detail a hex wrench today for a metric bolt, I'd estimate the opening tolerances by converting the size to inches, and using the tolerances from the nearest sizes of inch wrenches to estimate a working tolerance for the metric wrench.  And then would probably forget to convert the tolerance back to mm, thus foobarring a perfectly good drawing.

RE: Bolt Hex vs Wrench Size

tlee123:  International standard tolerance on wrench and socket wrench openings (ISO 691) is as follows, where s = fastener hexagon (and bi-hex) nominal width across flats (mm). Symbol "<=" means "less than or equal to."

          ----- Tolerance (mm) ----
          min    max (tol  max (tol
                 class 1)  class 2)
 3<=s< 4  +0.02  +0.10     +0.14
 4<=s< 6  +0.02  +0.12     +0.16
 6<=s<10  +0.03  +0.15     +0.19
10<=s<12  +0.04  +0.19     +0.24
12<=s<14  +0.04  +0.24     +0.30
14<=s<17  +0.05  +0.27     +0.35
17<=s<19  +0.05  +0.30     +0.40
19<=s<26  +0.06  +0.36     +0.46

For open wrenches, only tolerance class 1 is used. For closed wrenches and socket wrenches, manufacturers are free to choose either tolerance class 1 or 2. I don't know which class is more common; I haven't checked.

RE: Bolt Hex vs Wrench Size

ANSI metric screws and bolts are in Machinery's 22nd Ed on pp 1070-1080

Good Luck
To get the best from these forums read FAQ731-376 before posting

Steam Engine enthusiasts:

RE: Bolt Hex vs Wrench Size

Sorry guys, speed reading only works when you can concentrate.
My Machinery'shows wrench opening tolerances on page 1435. the Twenty Fifth Edition.

Next time I'll slow down when reding posts.
Bob Creely

RE: Bolt Hex vs Wrench Size

Oy, time to update my copy of MH!

RE: Bolt Hex vs Wrench Size

vonlueke - I couldn't find that information (metric wrench opening tolerances) in the MH I just ordered.  Is it in there?

What I did find in the MH for inch wrench opening tolerances is the methodology used to calculate their numbers.
Min opening = 1.005W + 0.001in
Max opening = 1.01W + 0.005in
W is the basic width of the nut flats (ie max nut size)

This seems to be a little different from the ISO metric standard vonlueke posted.

RE: Bolt Hex vs Wrench Size


now I'm wondering too.  We've got the 24th ed., and it doesn't have metric wrench opening tolerances.  BTW, the 19th ed. on my desk has the ISO metric bolt head tolerances, and they didn't change relative to the 24 ed.

The inch formula works (iff you convert from mm to inches, then back again), but gives an opening that is within (i.e. tighter than) the metric "class 1" tolerances.

RE: Bolt Hex vs Wrench Size

Totally unrelated to the original question, but I have to throw in a recommendation on Machinery's Handbook.  The electronic version somewhere along the way became a .PDF file, and now it's actually FASTER to use than the paper one.  I copied the whole thing on my hard drive and set a shortcut to it, and now I use that all the time and the poor book just gathers dust.


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