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