goofy thread pitch

Maybe there was some popular lathe in 1901 that had change gears that made it easy to get 27 TPI.

1/16-27 Straight pipe maybe? That's a 5/16 OD Pipe.

IIRC, gas illumination pipes used 27 TPI. And because the gas pipes were later used to run wires for the electrical fixtures which replaced it, things like the lamp sockets were made to screw onto the same thread. And -- they still do today. :-)

That thread was probably originally picked to be something which the home user could not cobble up -- after all, the illuminating gas was dangerous. :-)

And -- the same pitch (in 5/8" diameter) is used as the common US based microphone mount thread.

Makes me glad that my 12x24" Clausing includes 27 TPI (and its mutiples and sub-multiples) in its quick-change gearbox. (Not all lathes do.) Of course, the basket-of-gears change-the-yourself lathes could usually be set up using the standard gears which came with the lathe.

Enjoy, DoN.

That's a pipe thread. I've also wondered how that came to be. Another wierdo is the thread used on garden hoses and sillcocks, 1-1/16 x

11.5 TPI. My lathe can do both of these threads.

5/8-27 thread is also used in CATV line amp housings. The idiot manager at the cable company I worked for paid over $250 for a tap to chase aluminum from the stainless steel inserts when some cable monkey would over tighten a connector and strip it. He was bragging about the bargain, and told me there was no way I could buy one for less. I made a trip to my favorite surplus store and bought one for $7.50 that still had the protective coating on it. Boy, was he pissed! ;-)

gas pipes- weird.

This seems to explain some 1" diameter round and hexagonal dies I inherited that never seemed to fit anything. It seems they're really this

27TPI and fit 1/8" pipes (just checked). They are tapered too, which I never noticed.