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Witworth Sizes

Witworth Sizes

Craig Sawyers

Lou Danzico wrote:

Does anyone know how Witworth fittings are measured? I need to replace one and don't even know what to ask for.

Are they, 1/2", 3/4" or 4mm, 5mm or some other system. Also, how do you identify the no of threads, on a per inch basis, per mm basis, etc.

It's the little things that get you.

Whitworth was an older system, before introduction of UNC, itself superceded by metric.

TPI for Whitworth as follows:


So the only difference here is the 1/2" spec, which is 13 TPI for UNC. All other TPI the same in this range of bolt diameters. So it is difficult to tell the difference measuring TPI.

There are differences in the minor diameter (i.e. the thread depth is slightly different), because the thread angle is different (60 degrees for UNC and 55 degrees for Whitworth).

The corresponding fine thread guages are BSF (the fine version of Whitworth) and UNF (the fine version of UNC).

You can buy a tool called a thread guage, which looks like a set of feelers, but each leaf is cut on its edge with the thread form. You hold it against the bolt in question, and if you can see light leaking between the feeler and the bolt, you've picked the wrong thread.

Most good nut and bolt suppliers should be able to do both types. In context, I don't know when (or if) Jag swapped from Whit to UNC. My V12 is a hybrid, with imperial (UNC/UNF) block and metric heads. Astonishingly, that's the way it came out of the factory.


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