On Fri, 11 Jan 2008 20:07:11 -0800 (PST), "
wrote:

On Jan 11, 7:28 pm, "Don Young" wrote:
Going through some old taps I found a tap marked 1/4"-18. I know that is the
designation for a standard NPT tap but this tap is actually 1/4" in
diameter. Looks like a 1/4"-20 with coarser threads. Does anyone know of an
application for such a thread? I suspect it might have been made mistakenly
from a list of sizes which included the pipe thread. No manufacturer
identification and appears to be a cheap import quality.

Don Young

A quick Goggle search turned up a web site listing 1/4-18 straight
pipe tap (NPS) using a 29/64 drill bit for the hole. So, I guess it is
a regular size tap, just for a little used application.

Years ago, I had to turn some pipe fittings for a vacuum pump my late
business partner was going to use in his chemistry lab. The threads
were like yours, except about 3/4 as I recall. Same thread as 3/4
pipe, but no taper. The fittings sealed flush to the top of the pump.
Taper was not used for sealing. Maybe your tap was for something
similar.

Paul in Central Oregon

Apparently McMaster Carr carries them. They were used on some old
machinery, apparently - as well as on some thermocouple applications.
I think it is also used on some electronics devices - the thread on
some control stems (potentiometers) is 18 TPI from what I remember - I
know neither UNC or UNF nuts fit, and nor do metric standards.

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