[SIGP2101]

Originally Posted by postino

Whitworth doesn't list 30tpi for *any* size screw thread...

I see your problem, that is funny because i found this on the Inet.

English threads are specified by how many peaks there are in one inch of the length. They are specified as "Threads per inch" written "TPI" The diameter is specified in "thousandths of an inch" The most popular English threads are: 48 TPI, 40, 36, 32, 30, and 24 TPI

Although English and Metric threads are not interchangeable, for the purposes of identification they correspond M-.5 : (48tpi) M-.75: 32TPI M-.9 (30tpi) M-1.0: 24tpi. (These are not necessarily the closest English equivalents but the closest commonly used English threads.) Metric bashers refer, for example, to an M6-1.25 as "A Metric Quarter-Twenty".

Than this

The thread pitch can be gotten by using gauges or by using known screws as gauges (even if they are different diameters) Hold the known screw agains the one to be measured to see if it fits. A very slight mismatch means you are probably using the wrong measurement system. Metric instead of Enlish, or vice versa. It is a common suprise to find older items of European manufacture made to English specification for the American Market.