Posted by Carl Wilson 4 on 11/01/2017 20:05:54:
According to the book I'm consulting Gas Threads (Brass Pipe Sizes) everything from nominal size 3/8 to nominl 1" is 27tpi.
Carl,
Is your book of US or UK origin? I have just consulted my copy of Machinery's Encyclopedia (Published in New York in 1917) and it states:
Gas Fixture Threads. – Thin brass tubing is threaded with 27 threads per inch, irrespective of diameter. The so-called "ornament brass sizes" have 32 threads per inch. The standard sizes of the thread are 0.196 inch (large ornament brass size) and 0.148 inch (small ornament brass size). There then follows a table of "Gas Fixture Threads (Brass Pipe Sizes)" giving nominal diameters, actual diameters, and pitch. There are three nominal diameters given below 3/8", starting with No.4 (0.246" diameter) followed by 1/4" and 5/16".
I presume that these threads were used by the American manufacturers of gas fittings (lamps and the like). I also presume that the equivalent British manufacturers used the British Brass Pipe thread of 26 TPI.
The British Standard Pipe series is also listed, as is the American Briggs Standard for taper threads, ASME standard for parallel pipe threads, hose coupling threads…
It is interesting to note that the contents of the table JasonB linked to are very similar to the thread section of Machinery's Encyclopedia, including the obscure watchmakers' threads and the reference to CEI threads (which became the British Standard Cycle series at least as early as 1950). I suspect that the compiler of the table taddled most of it from the Encyclopedia or its close relative.
George
Edited By Georgineer on 12/01/2017 01:09:20