Mystery thread gauge

I have just come across a thread gauge in my workshop that is a complete mystery to me. The case is marked "Whitworth 55deg." and the blades match a conventional one that I have. However, the blades are marked 12G 1/2" 9/16" or 18G 5/16" and so on. Can someone tell what it is for please?

Regards

Maurice

It is the ISO designation for BSP. G simply indicates when that BSW size is the same pitch / TPI as a BSP (parallel) thread. By coincidence this letter is the first of the old name for pipe thread - gas thread - although i'm not totally sure if it is completely the same as BSP. If i'm wrong on these someone please correct me!

1/2" and 9/16" Whit are 12tpi, 5/16Whit is 18tpi simples.

J

Hi Ralph, I think you are wrong on all accounts. As far as I'm aware there is no ISO designation for Whitworth or BSP threads and they remain a British Standard. Gas threads were so called, I think, because they became a standard when pipes were used for gas lights and appliances and are exactly the same TPI as BSP. 1/2" BSP/Gas is 14TPI and there appears to be no standard for 5/16" or 9/16" pipe thread. It is not un-usual for thread gauges to have a G next to those that are standard Whitworth bolt thread. Not sure if the G means standard gauge.

Regards Nick

Hi Nicholas, it is surprising how you can learn things that are wrong. I had wondered because I know there is no 5 and 9/16 BSP, but had thought this to be just an example of its use so ignored it. If you look up this "G" thread note on the internet it has been asked before and many give the answer I have given, so it may be a common misconception. More thought on the subject, as you have put into this, shows it o be incorrect, as you say on all accounts, so thank you.

Try ISO 7/1 for Whitworth form threads, albeit these are for pressure tight (BSPT) rather than BSPP threads.

For internal parallel threads the designation is G(size), eg, G3/4 is an internal thread for a 3/4" pipe, parallel thread. For an external thread the designation is G(size)(tolerance), eg, G3/4A is an external parallel thread for a 3/4" pipe, tolerance class A.

Andrew

This must have been what I was thinking of in part at least. Quite familiar with Gs and Hs on tolerances but didn't know the connection to these.

Hi Andrew, thanks for pointing us in the right direction. I got the reason for calling them "Gas Threads" correct, but didn't realise that it originated from Germany though.

Regards Nick.

I think that internal and external parallel threads are designated G (German for gas is gas!) but for tapered threads the designator is R (German for pipe is rohr).

Like all standards the one thing you can say about it is that it isn't.

Andrew

And just to confuse us all even more this chart has 5/16" and 9/16" Gas thread, not to be confused with Whitworth pipe or BSP

For a reasonably convincing explanation ... see here

"Gewinde"

MichaelG.

Edited By Michael Gilligan on 13/06/2015 09:58:33

Thanks Michael, confirmation of what I said that the G## was the tpi

Strange that they only put the dia on the less common sizes, maybe at the time they were first produced everyone was expected to know the tpi of the common diameters.

Edited By JasonB on 13/06/2015 10:00:13

.

It's maybe worth stating the obvious:

The blade-type gauges only check tpi [or pitch, for metric] ... i.e. they do not tell you anything about the diameter, and cannot 'identify' a bolt or fitting.

The same "Whitworth" gauge checks BSW, BSF, or any arbitary diameter with a thread of that form.

MichaelG.

Just to complicate things, the French have tried to metricate BSP thread sizes and made about as good a cock up as the British! The British worked on Iron Pipe dimensions and the French have worked on the size washer you would need to connect a male to a female fitting. follow the following link :-
**LINK**

BobH

Bob, the second answer on that page seems to suggest its ID and OD of the pipe to the nearest mm not the washer?

Can't see how you could get a 27mm OD fibre washer into a 3/4" BSP threaded nut as the minor dia of the nut is 24.1mm

Edited By JasonB on 13/06/2015 11:22:43

I'd use a hefty dose of BF & BI.

Andrew