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

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.

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

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

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

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.

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

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

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