" wrote in message
...
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


Pipe threads are not to be confused with other types of threads, be they
straight or tapered. They are unique to pipe, and pipe alone. A 1/4"
pipe has an OD of .540", far greater than the tap in question. Note that
the tap drill size you mentioned (29/64") is .453", much larger than the
major diameter of the tap in question.

Harold