I believe something like this has been mentioned before. The G is a reference to the German word for thread possibly Gewinde. 13tpi is American 1/2" UNC.

But there is no 13tpi on BSP or G threads

Hi, many Whit gauges include a 13 G one and while there are no standard Whit threads that are 13 TPI, 1/2" whit and 1/2" UNC threads can look similar especially if you don't have one of each to compare, so a 13 G one is handy to have to tell if you have a UNC one for sure.

Regards Nick.

With today's cheap thread pitch gauges made in China, anything is possible. They quite possibly make the same gauge for BSW and UNC and just change the labelling. The gauge is for counting TPI not determining flank angle so they may well use the same gauge for both, which would mean a 13TPI to cater for 1/2 UNC ending up in both sets.

I have a set of feeler gauges of reputable mid-range quality brand (Kinchrome) made in China like everything these days, and a check with a micrometer (genuine Mitutoyo 40 years old)  reveals that many of the feelers are not the thickness marked on them. EG, the 2, 3 and 4 thou feelers are all the exact same thickness. Others are not the exact imperial thickness labelled, but the nearest metric equivalent. So, like I said, anything is possible these days with cheap tools or even not-so-cheap-to-purchase but cheaply made in China.

Edited By Hopper on 16/06/2022 06:34:13

In a similar vein I have been told by a retired GPO Telephones engineer that the organisation preferred 3,5,7 BA screws etc to the more common 2,4,6 as it meant that fewer were taken for 'home' use.

The OP doesn't say the gauge is 13tpi. The letter G usually refers to BSPP threads. Sometimes the nominal imperial size is approximated in millimetres. So I expect the gauge is for 1/2" BSPP. May be the OP can tell us if the gauge is 14tpi?

Andrew

The metric system is not rational but arbitrary, the same as any other thread system (except possibly BA which is based on a geometric sequence). Before, and during, WWII the French and German metric thread systems were different below 6mm OD. For instance the French used M3x0.6 rather than the international standard 0.5mm pitch. Use of M3x0.6 is still part of the JIS system in Japan, as I discovered when I made some new parts for a guitar for a friend of a friend last year.

Andrew

The symptoms of Hopper's 'reputable mid-range quality brand' feeler gauge strongly suggest a counterfeit. It's not reputable, mid-range, or quality!

Counterfeit goods haver been around forever, and aren't specifically made in China or a modern phenomenon. Birmingham, England was once famous for cheap fakes, as later on were the USA, Germany and Japan.

Sadly, there's always someone, usually in a developing economy, who is prepared to make a quick buck out of making fakes. Worse in my opinion are folk in the developed world ready to dishonestly sell them on. Most of the profit is made by them, not the manufacturers, so the master criminal who needs a damned good flogging might be your next door neighbour, not a disreputable foreigner.

Unusual to find cheap fakes being made in developed countries because there's not much profit in producing them, unlike selling to victims!

China will follow the usual pattern: as wages and standards rise, fake making will move to other countries new to manufacturing.

Dave

But those foreign johnnies would struggle to work out the standard Whitworth system anyway, being born and raised on the entirely rational metric system!

At my previous employment we reworked a wartime built (WW2) German crankshaft grinder where all the threads were Whitworth. The machine came with all it's original documentation, with b & w photos stuck to type-written pages. From the number of throws on the mounted crankshaft in the pictures of the set-up machine + the size of the machine, I supect that these were U-boat diesel engine crankshafts. The machine most likely came to the Uk as reparations.

The gauge is for counting TPI not determining flank angle

I disagree – it is quite possible to determine the flank angle from thread gauges when held against the light. I have Unified, Whitworth & metric gauges at work for identifying threads, with the difference in "sight picture" on UN & WW threads of the same TPI being pretty obvious.

Nigel B.