Over / under size nuts

Recently I bought a few tubs of the contents of someone else's workshop from an antique / house clearance centre.
One of the items was 14mm M8 nuts. I bought these to use on coach bolts as I normally use penny washers to reduce the indent into the wood, and thought these might be helpful.
This got me thinking about nut sizes, when might none standard size nuts be used and does it make much difference? Whitworth sizes were reduced after the war to save steel/money and one size smaller BA nuts are often used because they scale better. In the recent edition of ME the use of retapped nuts was discussed for the same reason. How much does the size of nuts matter? How small can you go without risking mechanical failure failure? Or does it not matter because the male thread is the weak point?
Although I have referred to nut sizes, equally how important is the bolt head size?

It depends a lot on the strength of the material, Keith

… remember that Allen [hex socket head] screws effectively have very small ‘spanner size’ and [provided the quality is right] that all works fine.

MichaelG.

Undersize and strange size nuts and bolt heads are sometimes used on motorcycles, particularly where there is restricted access.
It also allows a smaller collection of tools in the supplied toolkit, though that might not be the main reason.

e.g. in metric I'm used to 10,13,17,19mm spanner sizes for 6, 8, 10, 12mm threads, but several Japanese bikes use 12 &14mm spanner size, for both under and over size headed nuts & bolts.

Bill

Motorcycles - Funnily enough Triumph / BSA cylinder base nuts spring to mind.. ..the finer the pitch, the smaller the nut can be;

..is it the case that the threads are under tension whilst the wall of the nut is in compression..? .'. the threads will fail first every time?

The objective of a threaded fixing is to apply a load to clamp the two items together.

The size of the hexagon (IF it is a hexagon; there are square, octagonal, even triangular or pentagon heads used for various purposes, not to mention the various socket types, such as Allen, bi hexagon, and proprietary such as Teac ) is to provide a means or rotating the threaded fastener to apply that load, or to provide an adjustment.

No need to go into fasteners requiring C spanners or tommy bars here.

On some Renault 5 engines, to improve access, the M8 nuts clamping the carburrettor to the manifold were 12mm A/F rather than the usual 13 mm.

The hexagon, or whatever, is merely the means of rotating the fastener to apply the clamp,load. The sizes are standardised to make life easy for everyone.

Howard.

Thanks for all of the replies. For some reason the reduction in number of spanners didn't occur, even though i was aware that some used Whitworth heads on other systems due to Whitworth being the spanners in use at the time.
Howard, I do have a collection of C spanners, one stamped Rolls Royce. I do remember seeing someone using a hammer and punch as an alternative to a C spanner, many decades ago.. Curious where triangular and pentagon ones are used unless it is to trap them in that shape or to prevent their removal unless you have the correct spanner.

Oversimpliying. In the limit the wall thickness around the thread only needs to be great enough that the thread shears off at the base before the wall cracks and shears.

Given the very non-linear load distribution associated with "pin in a hole" configurations where most of the load is carried close to the open end odds are that a wall thickness of around 3 times the base width of the threads will suffice.

Out in the practical world it seems extremely rare for a threaded pipe or tube to fail before the thread itself. Some of the standard pipe and thread configurations leave what, to the naive observer very thin walls.

Clive

Much thought and practical experience has gone into the design of the humble nut and bolt. Early heads were square because they and the spanner are both easy to make by hand, but the form is weak, the corners easily damaged, and the sharp edges gouge the material underneath. And only having 4 corners makes it harder to fit a spanner in a confined space.

Hexagon heads are better, the form supports the shoulders, the corners are harder to damage, and 6 edges are less likely to gouge than 4. Nonetheless, hex heads are relieved to reduce the risk, which unfortunately reduces the bearing area, which is bad news for soft materials, hence washers!

Whitworth was much influenced by the materials of his day. In the 1840s engineering materials were all relatively soft: cast-iron, wood, iron, wrought-iron, brass, copper, stone and brick etc. Steel was extremely expensive and unlikely to be used for ordinary fasteners. Does anyone know what Whitworth Nuts and Bolts were originally made of? My guess is mostly Iron until Bessemer steel changed everything 30 years later. Anyway, Whitworth's oversized heads made perfect sense when Queen Victoria was a teenager, but were an anachronism well before 1940 by which time steel bolts and washers were almost universal.

Although most fasteners today are standard there are plenty of good and bad reasons why a manufacturer might choose to be different. On the side of the angels tamper-proof fasteners are used to stop people undoing the wrong fixing, but they're also available to Satan to prevent simple home repairs. Cost cutting during manufacturing is another reason: on a production line, it might well be cheaper to fit a robot with a single socket and have it tighten a range of different bolts with non-standard heads. Mass-production has it's own logic, and they rarely care about men in sheds!

Bottom line though, the bearing area of a fastener should be big enough to prevent damage, including failure, of the material it's holding. The answer is usually a washer.

Dave

commercially available Hex Nuts in the US are

1-1/2" screw size > 2-1/4" across the flats Nut size, i.e. 2.25/1.50 = 1.5 ratio

3" screw size > 4-1/2" across the flats Nut size, i.e. 4.50/3.00 = 1.5 ratio

I would be tempted to use the 1.5 ratio on model work for smaller size Hex Nuts. In my professional design work I often used "narrow pattern" Hex Nuts for mechanical assemblies. For #10-32 (0.190" major dia) thread size, Hex Nuts are available in both 1/4" and 5/16" across the flat sizes. I never encountered one of these Nuts failing.

I would think if you took the (Major dia minus the tap drill size)/2 as the approx thread depth, add two of those thread depths to both sides on the screw size (total of 4 x thread depth) you would get a satisfactory minimum Hex size for model work. In the case of the #10-32 screw size -

(0.190 - 0.159 ) / 2 = 0.155" ANSI #10 Thread major diameter, Tap Drill #21 = 0.159" dia

0.190 + (4 x 0.155) = 0.252" which matches the 1/4" across the flats size listed above.

with the 1.5 ratio,

1.5 x 0.190 = 0.285" or 9/32" which lies in the middle between 1/4" and 5/16" sizes listed above.

In any case of a design for anything in real life I would not stray away from the commercial sized components. And remember there are many different strength grades for Nuts. I remember rebuilding a Triumph Spitfire engine in the early 1970s. The Manual stated not to re-use the Hex Nuts that came off the Studs holding the Head on. I, being young and dumb, decided I knew better and re-used the Hex Nuts. Half of the Nuts failed as I approached the torque value listed in the Manual. The Studs did not fail. The original engine designers made a conscience decision which item was to fail first and selected the material properties to suit the application. I am sure this was based on previous design experience and testing. I think we ended up buying the new Hex Nuts at the Auto parts store as genuine Triumph spare parts.

Here, where I live, northeast Ohio, the Water shutoff near the street is covered with a ~4" dia round cast iron Cover Plate which is secured with a 5-sided screw which is recessed in a counterbore in the Cover Plate. This is to make it tamper-resistant.

Bruce

I recently found brand new engine retaining bolts with 17mm a/f heads and mating nuts which were supposed to be nyloc type!! 16mm a/f ,the nylon bush in the nut had a too large internal diameter so had little grip on the bolt thread and could be removed usings ones finger,never known that problem before,no doubt chinese origin.

Some old British cars (Talbot) had metric threads with Whitworth heads.

Landrover were quite clever, I believe. Bolt heads and nuts, for those bolts, were often of different A/F sizes as I recall. This allowed the items to be removed/replaced with a single set of spanners.

The original Fergie tractor only needed two spanners for all regular maintenance.
Whitworth when specifying fastenings also specified the length of the spanners so that the average bloke couldn't get enough leverage to overstress the bolt.

Not so long ago one of our designers had the bright idea of making the screws on one of our products torx so that customers would be less likely fiddle with them. Only later twigged that as we had thousands of technicians who might legitimately need to undo them they all had to be issued with the appropriate tool.

One thing to remember is that it is not the body of the nut that is normally critical: it is the area that it is being tightened onto. To save weight, aircraft nuts are often reduced to a minimum and have a flared bearing area. Besides saving weight, these have a second advantage. By distorting the hex slightly, this gives them a self-locking effect, too.  (Photo from Demon Tweeks)

It is not just the nuts that can benefit from weight saving. The bolt, below, is a typical aircraft tension bolt. It also has a 12-point head to allow a higher torque to be applied.

Edited By DrDave on 04/11/2022 10:14:54

For anyone wondering where triangular or pentagonal heads are used.

The end fittings for the Gpvernor Pivot Shaft on nthe Bosch EPVE and its sublings were triangular. A socket to mfit could be purchased.

No prizes for guessing the supplier!

The cassette type injection pumps supplied by Diesel Kiki used pentagonal fixings.

(Found this out shortly after assuring a potential supplier of hardware that we would not want anything odd, like five sided heads, just ordinary hexagons! )

Howard

Most farms probably still have "fergie" spanners. They had an inch ruler of the body to aid measuring.

NDIY you are quite right 5/8 x11/16, 3/4 x 13/16 are some that I remember

Some current foreign manufacturers seem to use non-standard metric hex sizes for nuts. A very widely sold 3D filament printer I bought a few years ago had M8 nuts on the holder for the reel. The printer was supplied with a skinny sheet metal spanner, which tightened the nuts OK, but I found that my usual13mm spanner wouldn't fit. Measurement revealed these nuts had a 14mm hex. This was a very lightly loaded pair of nuts, so I can't see a reason for the extra meat. In the end, the inconvenience resulted in those nuts being consigned to the bin, to be replaced by standard 13mm hex M8 nuts. I'm still puzzled, though.

Marcus

14mm AF for an M8 nut is quite commonly met with. An M8 58 piece clamp kit I've got has 14mm AF flange nuts. You'll also find 12mm AF for M8. I've got some 12mm AF M6* nuts [from a toilet seat fastening kit], M10 nuts with both 16mm and 17mm AF, and M12s with 18mm, 19mm and 22mm.

See my photo below, showing commercially made 12, 13, and 14mm AF M8 hex head set screws.

I made some 11mm AF M8 nuts a while ago for use in confined spaces on an angle plate. I think having a variety of AF sizes is good.

*Correction: the M6 toilet seat nuts are 13mm AF!

Edited By Bill Phinn on 05/11/2022 18:02:06

One of my old (semiretired) mowers has unified sizes on the engine and metric elsewhere. I assume because B & S stuck with unified, but Metric was used for the rest to make export to Europe easier.