I received the following information via email. I believe it was originally posted to the Jag email list. I found it quite interesting and thought I'd retain it for posterity. Used here with permission.

Arranged and Formated for the SOL Morgan web page:
By: John T. Blair (WA4OHZ)
Va. Beach, Va. 23464; (757) 495-8229

NOTE: This appears to be written from a British prespective.

Last updated: April 04, 2000

In the later half of the second World War all British Military vehicles/ Equipment were changed and were made using Unified Fine and Course (UNF/UNC) threads. This was because of commonality of spares to support the massive amount of equipment being given to the war effort by America. For UNF/C read ANF/C There were only very small technical differences in tolerances.

However Our national standard since the 1870's were British Standard Whitworth and British Standard Fine (BSW / BSF). These are similar to UNC/U NF but the main differences are the BS use a 55 degree flank and the UN a 60 degree, as do the current ISO Metric threads in Europe. However a 1/2" BSW has a different pitch than a 1/2" UNC as do most of the BSF series. I would strongly recommend that you do not cross mix threads as they will be 20% weaker due to the different thread angles at the very best! Most coarse UNC / BSW threads are used into aluminium and this is where you will have problems with stripping and corrosion particularly with Stainless steel bolts.

All UN material Stock sizes used to carry a row of circles pressed into one face of the flats, but this has not been maintained due to imports. Another indication [but not foolproof ] is that if you measure a standard UN nut it will be exactly a decimal equivalent multiple of 1/16" size across flats. i.e. .4375", .500", .5625", 625", .750" etc. Whitworth / BSF will not normally be so. e.g. .525", .600", .710", .820" etc. accross flats. This is why the ZZZXXZZZ spanners / wrenches never fit ! MOST major UK car makers used UNF/C threads on NEW models after the war until the early 70's when we started to change to metric. This was due to a 1965 Government recommendation to convert to UN metric as 1st choice and UNC/F as second choice. However the use of BSW/F carried on for Lawnmowers, Garden equipment and maybe some small car producers until the 80's when most went metric.

Although our favourite Marque, Jaguar, was a low volume producer. Often they did not have the purchasing power, I suspect, to fully tool a part and so would have to use other manufacturers components. As part of British Leland, they were never really integrated. [I smile when I see all the worthwhile effort going in to reconstructing the build standard history, but stongly suspect that as Jaguar were building for a changing market requirement in the US, Australia and other places and coping with the supply problems we inflicted on ourselves at that time, the exact build standard was made up "on the line" from what was to hand to meet the various technical standards !]

Other threads

Pipe Connections
These used BSP threads. They were basically Whitworth 55 degree form but were nominated by the bore of the pipe. i.e. 1/4" BSP was about .528" O/D and 20 Threads Per Inch (TPI). This is why 3/16 Whit spanners often fit brake pipe unions etc try it!.

Electrical Fittings
Like the small size 4-40, 6-32 ANC/ ANF etc, we also had small screws for attachment. These were called British Association (BA) and used a 47 and 1/2 degree form. They use a number sequence and had their origins based on metric threads used in Europe for instrumentation. Basically most of Industry used the even numbers, 0, 2, 4 etc whilst the household electrical industries used the odds 1, 3, 5 etc. The thread number specified the size AND the pitch. There was no coarse or fine. You may find 2,4,6 BA threads in Lucas equipment built before the 80's. They are still freely available here.

There are other thread systems e.g. BS Cycle - these have Whitworth 55 degree forms but nearly all have a constant 26 TPI except for the very small sizes. They some time crop up on old Motor Bike but not to my knowledge on cars.

Is stainless rustproof? Yes and no. It becomes rustproof by the formation of a very thin oxide on the surface (as does aluminium). This is electrically non conductive and hence removes the basis for corrosion. If you scratch it and eliminate oxygen, e.g. keep it under water or oil, to prevent the formation of the oxide, it will rust. As it does sometimes under washers. Certain chemicals e.g. chlorinated products will also cause corrosion on stainless steel due to their reducing nature. If you use passivated Zinc plated fastenings these may prove better as on steel they are sacrificial as the plating dissolves to protect the bare steel.

(ED. Mike sent me the following additional information about rust, based on my question:
I must admitt that I don't understand what you are trying to say in the paragraph about stainless steel. I thought rust was oxidation. However, you infer it isn't.)

Re corrosion. This is called RUST on steel but all metals corrode.

What happens is when an Electrolyte, say salt water, lies on bare steel, two adjoining crystals are connected by the electrolyte. These form a battery! this is because of the different characteristics, chemical make up and structure of the two crystals. This causes one, the lower or cathodic potential one to oxidise creating rust on steel or pitting on aluminium, Painting stops this but paint layers are porous so many are needed.

Certain materials, aluminium, bronzes, Stainless steel all form on oxide which is electrically insulate [but no air/ oxygen then no oxide forms], These Oxides stop further corrosion taking place unless some form of other chemical agent breaks down these passive oxides. e.g. sulphurous environments from industry attack Anodised Aluminium.

Anodised Aluminium has had a thin film of very hard insulative oxide formed on the surface which is a protective finance. Try an electrical resistance meter on it ! IF you push the probes hard you will break through the thin oxide [ about .0004" thick] and get a reading. Aluminium does not therefore make good electrical contacts [in most environments] despite being a good conductor!

Stainless steel will rust if oxide is kept from it and it is scratched. i.e. rust stains under stainless washers are common and the mild steel around the hole can also rust worse for reasons explained below.

Oxidisation of plain steel. This actually can form two oxides: FE3O4 and FE2O3. The former is red rust and will keep forming until the metal is all converted. The later can be formed by adding certain acids to the former and converting it to a BLACK oxide which is insulative and hence forms some degree of protection. This is how the Genolite/ CureRust type of products work. You then paint over the BLACK Oxide to give it further protection... OK IF you are sure you got it All!

Zinc plated bolts are special. In a salt laden environment, the zinc coating very slowly dissolves and the free ions of zinc attach to the steel adjacent to the bolt thereby protecting it with a kind of Zinc plating. For cars they may last typically 20 to 50 years but it will depend on the climate.

Large Boats have copper based [Bronze] stern bearing tubes so they have a different problem. The steel is cathodic to copper in salt water so the steel ionises away trying to "steel plate the copper. Result, large rust hole around stern tube bearing. Large ingots of Zinc are often bolted to the back of the hull to dissolve away, protecting the steel hull and neutralising the above galvanic reaction.

So do we rebuild our cars with SS or zinc plated bolts. FWIW, my view and it is only personal, I would be happy to use SS if I lived in CA or Texas, Spain or Italy. I would personally use Zinc plated if I were to use my car mainly in Mm, Chicago or Montreal.

Hope this helps to make a very complex subject simply a bit clearer.

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