Why are hex head bolts hex rather than Octagonal (or square?)

Whilst thinking about another problem, I find myself wondering, "why is it that bolts have hex heads rather than octagonal or some other number?"

Is there some wisdom about the history of this?


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According to Bryan :

Well ... for a few guesses:

1) Hex has more surface area on the flats, and is harder to round off with a wrench. 2) Hex uses less material relative to the major diameter than octagonal. 3) Hex stock stores more compactly. No waste space. 4) It takes fewer cuts to machine the flats, if not working from stock already that shape. This saves machining time. 5) Hex is easy to grip with a three-jaw chuck, which is the best for quick gripping of round stock. Octagonal would require a universal 4-jaw -- which does exist, but which is uncommon, and a slight irregularity with 4 jaws means that one will not be gripping firmly.

I'll leave others to deal with the history. I've just listed the advantages to hex which come to mind readily. Besides -- it is time to go to bed. :-)

Enjoy, DoN.

Reply to
DoN. Nichols

First off, ( and most obvious ) if it had anything other than 6 faces, wouldn't make much sense at all to call it a "hex"......

Eight points tend to strip too easily...

And at some point, needing to tighten fully to 90 degres in order to be able to reposition a wrench was apparently too much for design engineers...

I kinda like the NAS style aircraft fasteners, personally.....

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A guess: bolt heads were once square, requiring 90 degrees of rotation before a simple wrench could be repositioned to grab another bite. Machinery got more complex, spaces got tighter and access more limited, hence hex requiring only 60 degrees of rotation between bites before robust ratchet wrenches were available.

It's easier to strip the corners off a hex than a square, but the hex is usually strong enough to shear a bolt before the corners fail.

Reply to
Don Foreman

Design engineers usually do what marketing directs because marketing usually controls engineering funding.

Who do you suppose designed those fasteners? Might it have been design engineers allowed to design for peformance?

Reply to
Don Foreman

Here is my answer: Hex once were square (and not called hex :-). They used to much space (distance bolt hole - housing). The next step would have been a pentagon. One one huge pentagon was built (some in there are nuts), but a pentagon is not something you could grap with a wrench. So a hex, the next step, is the closest to a circle (least assembling space) and the nearest to a square (best contact surface for a wrench).

HTH, Nick

Reply to
Nick Müller

Note that the typical open end wrench is angled---so by flipping the wrench over you can make small adjustments in confined quarters. It would not work as well with a square head.


Reply to
Harold and Susan Vordos

Square is the first logical choice, and is still in use. No doubt that was the start.

Next logical choice is double that or octagonal. Lots of work for those filing.

First steps a labor unions or cost analysis? Hex.

Seems possible. Just guessing.

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6) Because wrenches are harder than the fasteners, it's possible to double the number of points on the wrench - from 6, to 12. 12 is bigger than 8, and it's probably not possible to double from 8 to 12, given the common metalurgy.

So by chosing hex heads, it increases the number of index points on the fastener from 8 to 12, over octagonal.


Reply to
jim rozen

Well, an even number of sides are needed to allow for open end wrenches leaving out 3,5,7,...

4 is the first number that works but has a fairly large diameter and only 4 wrenching positions. Worked great for farm implements. Still a good choice at times. Still used. 6 is next. Must have been a good compromise on diameter relative to bolt shank, 6 wrenching positions, and works with 12pt bo and sockets to give 12 wrenching positions. 8 would have to be closer tolerance on bolt head and wrench. Likely would not work well with open end wrenches or polish calipers.


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Pentagons are used here as the utility companies standard for shut-offs, especially below grade. Presumably to stop the un-authorized actuation of these valves, either on or off.

Brian Lawson. Bothwell, Ontario.

Reply to
Brian Lawson

I hope GM or Ford doesn't monitor this group. They will start using nonasymetrical pentagonal fasteners on everything.

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Reply to
Robert Swinney

Maybe because you can lay out a hexagon on a circle with just a compass and straightedge? The people who first came up with hex head screws didn't have CNC machines...

John Martin

Reply to
John Martin

Double from 8 to 12?

I think not.

But it surely could have been accomplished with Reganomics.... :-)


Reply to
Jeff Wisnia

Doesn't work, you still wind up with an unused (0.14159 * radius) circumference.

A hexagon is a natural shape, unlike squares, octagons, etc. Ever seen a beehive? A hexagon is stronger than a square or octagon and less likely to round off than an octagon.

-- Regards, Curly


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Reply to
Curly Surmudgeon

Don't see many hexagonal rocks. Besides, I know lots of people with square heads...



Reply to
jim rozen

Yes it does. The segments that comprise the circumfrence of an inscribed hexagon are chords, not arcs. Each one is (0.14159 * radius)/6 shorter than the arc they span.

Only six points of the circle lie on the hexagon, at the six corners.

Thanks for making me think.

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Um, beehives?

Reply to
Dave Hinz

"Jeff Wisnia" wrote: Double from 8 to 12? (clip) ^^^^^^^^^^^^^ Half of 8 is 4. 8 + 4 = 12. Do you listen to the "third half" of the Car Talk show?

Reply to
Leo Lichtman

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