Whitworth or SAE?

Here is some general information that I gleaned from an old MGA Service Manual. This has to do with the identification of Unified Screw Threads which are standardized in the following manner:

• Nuts: By a circular groove turned on the end face of the nut or by connected circles stamped on one flat of the hexagon
• Bolts and set screws: By a circular depression turned on the head or by connected circles stamped on one flat of the hexagon

So if you see any hardware with those identification marks, you will know right away that it is a Unified Threaded fastener.

The MGA Service Manual also states that the Unified Threaded fasteners and the American Threaded fasteners, for all practical purposes are interchangeable and use the same wrench sizes.

Basically all of the standard bolts you see on a Norton will have the circular depression on the head of the bolt. The plain nuts will typically have a circular groove on the face of the nut. Special bolts and nuts that are made from hex stock will typically have the stamped connecting circles on one of the hexagon flats i.e. spindle nuts, fork top bolts, cylinder based nuts, aft rear fender/fairing mount bolt, etc. Unfortunately, not all hardware has these neat identifying marks. It is a shame that the Norton Parts books aren't like the later BSA and Triumph books in helping to identify what is used on their machines.

PeterJoe said:

• Nuts: By a circular groove turned on the end face of the nut or by connected circles stamped on one flat of the hexagon
• Bolts and set screws: By a circular depression turned on the head or by connected circles stamped on one flat of the hexagon

So if you see any hardware with those identification marks, you will know right away that it is a Unified Threaded fastener.

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Also, original Unified studs (such as the Commando '72-on cylinder UNC/UNF base studs) can be identified by the short unthreaded first section as the visible end of the stud.

L.A.B. said:

Also, original Unified studs (such as the Commando '72-on cylinder UNC/UNF base studs) can be identified by the short unthreaded first section as the visible end of the stud.

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I did not know about the studs being identified in that manner, but it makes sense that they would identify the studs. Not long ago I noticed that the top thread was missing on the threaded studs for the engine of a BSA 500 MX I am restoring. I had no idea that these were Unified Thread identifiers, or any kind of thread identifier for that matter.

What is the long stud on the intake side of the head that holds down the valve cover? It has two types of threads. Fine & course maybe ?? Surely not your average hardware store find...........


Tim_S

Tim_S said:

What is the long stud on the intake side of the head that holds down the valve cover? It has two types of threads.

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The rear rocker cover stud 067693 (NM18033) is 5/16" x 22 BSF - 26 CEI (BSC/Cycle).

Tim_S said:

What is the long stud on the intake side of the head that holds down the valve cover? It has two types of threads. Fine & course maybe ?? Surely not your average hardware store find...........


Tim_S

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Norton have come in for some flak in this thread (pun intended) but the different threads have different purposes and in general Norton have got it right in engineering terms. The intake stud for example needs a coarse thread for going into the soft alloy head and a finer thread for the steel nut on the other end.

Thanks guys,,,,,,I'm now in the market for a better thread measuring devise ,,,,Starret Co. comes to mind.


Tim_S

Gents,

There are lots of thread varieties. American UNC (Unified National Coarse), i.e. 1/4-20; Whitworth, i.e. 1/4-20; UNF (Unified National Fine), i.e. 1/4-28; and BSC (British Standard Cycle i.e. 1/4-26, as a common example.

What we usually think are UNC threads on these bikes are Whitworth coarse, which has a 55 deg thread pitch rather than a 60 deg UNC (and BSC) pitch. Usually the difference is not important, especially given the clapped out machinery originally used, and the 40 odd years since manufacture, but they are not the same.

Fine threads seem to be usually BSC.

Whitworth/BSC thread pitch gauges are cheap.

It was always common practice on old British bikes to use 26 TPI whitworth (cycle thread) on fasteners which screwed into steel or cast iron, and coarse whitworth 23 TPI for fasteners which screwed into aluminium. On my racer, I've found old out of shelf life titanium bolts with UNF threads off the Hornet Fighter, and used them all around the engine, and mounting brackets. I use bullet headed whitworth allen screws with the heads turned down at the edges on the cases, and some of the bolts are still metric or whitworth. It's a nightmare if you are doing a rebuild. You need to collect all your spanners and just keep juggling them to find the one that fits. It doesn't really matter because if you own a British bike or car, the first thing you must understand is that it is designed to drive you insane. If I have a problem with anything on my bike, I simply re-engineer it, and get a much better result, than they did in the factory.