Ed Huntress wrote:

"RoyJ" wrote in message
...

I have a delightful table together by Andy Pugh of Bodgesoc Industries,
Sheffield, UK showing 526 standard thread sizes up to 6" . Closest it comes
to yours is a 16-18 ASME Thread with .268" OD and 18 threads

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


1/4 - 18 is a NPS (National Pipe Straight) standard size.

--
Ed Huntress


Hardly, in the case of NPS, 1/4 refers to the nominal bore of the pipe,
the od is .540, whereas this tap has a diameter of 1/4"

Tom