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

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

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

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


Had a look in a cat and 16, 18, 20, 22, 24, 26, 27, 28, 30 and 32 tpi taps
in 1/4" size, were in common supply, once.

Tom

"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

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.

--
Posted via a free Usenet account from http://www.teranews.com

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

Some thermocouple fittings are 18tpi and fit a 1/4" hole.

--
Posted via a free Usenet account from http://www.teranews.com

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

"Tom" wrote in message
...
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

Ah, I read the post too quickly. Thanks for the correction.

--
Ed Huntress

Ed, not if the tap diameter is 1/4 inch.
Steve

"Ed Huntress" wrote in message
...

"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

Tom,
They are still available, but usually special order.
Steve

"Tom" wrote in message
...
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
Had a look in a cat and 16, 18, 20, 22, 24, 26, 27, 28, 30 and 32 tpi taps
in 1/4" size, were in common supply, once.

Tom

clare at snyder.on.ca wrote:
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.

I worked in the electronic field for nearly 60 years and never saw
a potentiometer with a coarse thread. The 3/8 ones were 32 and the
"modern" 1/4 are 40. The damn metric ones I don't know about. :-)
...lew...

On Jan 12, 7:48 am, Lew Hartswick wrote:
clare at snyder.on.ca wrote:
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.

I worked in the electronic field for nearly 60 years and never saw
a potentiometer with a coarse thread. The 3/8 ones were 32 and the
"modern" 1/4 are 40. The damn metric ones I don't know about. :-)
...lew...

A few months ago I had to make some custom mountings for
potentiometers and encoders, and I had to buy a special 3/8"-32 tap
for the stem. It was aweful to tap since it was so large but such a
fine thread. Took me 4 tries to get it right.

" 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

"Ed Huntress" wrote in message
...

"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


But not in this case.

Harold

"Steve Lusardi" wrote in message
...
Tom,
They are still available, but usually special order.
Steve

"Tom" wrote in message
...
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
Had a look in a cat and 16, 18, 20, 22, 24, 26, 27, 28, 30 and 32 tpi
taps
in 1/4" size, were in common supply, once.

Tom


Thanks to everybody for the information. If anyone ever needs such a tap,
you know where to borrow one!! I did notice that it had "CARD" on it so that
is probably the manufacturer.

Don Young

On Fri, 11 Jan 2008 21:28:50 -0600, "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 1/4 x 18 straight thread is NPSF (straight) DRYSEAL ...whatever that
means in english. it is a pipe thread.

in a taper that thread would be NPTF (taper) DRYSEAL.


Stealth Pilot

Stealth Pilot wrote:

On Fri, 11 Jan 2008 21:28:50 -0600, "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 1/4 x 18 straight thread is NPSF (straight) DRYSEAL ...whatever that
means in english. it is a pipe thread.

in a taper that thread would be NPTF (taper) DRYSEAL.


Stealth Pilot

I hope you read you pilot notes with greater concentration than you did the
original post? :-)

Tom

On Mon, 14 Jan 2008 05:56:18 +1300, Tom
wrote:

Stealth Pilot wrote:

On Fri, 11 Jan 2008 21:28:50 -0600, "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 1/4 x 18 straight thread is NPSF (straight) DRYSEAL ...whatever that
means in english. it is a pipe thread.

in a taper that thread would be NPTF (taper) DRYSEAL.


Stealth Pilot

I hope you read you pilot notes with greater concentration than you did the
original post? :-)

Tom
Got me a left hand thread nut off a dead 1/2 inch arbor and decided to
make me an arbor to fit the other end of the 1/4 HP, double shaft
motor. Started to cut the threads when something told me to verify
that I had everything set correctly, oh-oh, I'm cutting 20 tpi and the
nut is 24 tpi. Fortunately I had only done one pass; my arbor has a
funny thread every fifth one.
Gerry :-)}
London, Canada

Stealth Pilot wrote:
On Fri, 11 Jan 2008 21:28:50 -0600, "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 1/4 x 18 straight thread is NPSF (straight) DRYSEAL ...whatever that
means in english. it is a pipe thread.

in a taper that thread would be NPTF (taper) DRYSEAL.


Stealth Pilot


The way I learned it the "F" stood for "fuel". Things like gasoline
would disolve many of the old dopes used to seal NPT threads and allow a
"spiral leak" along the threads.

The male and female tapered dryseal thread forms are designed to make a
liquid tight metal to metal seal when tightened.

I've never heard of an NPSF (straight) dryseal thread and it's sort of
hard to see how it could work, nor did Google help me find one.

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.98*10^14 fathoms per fortnight.

Jeff Wisnia writes:

I've never heard of an NPSF (straight) dryseal thread and it's sort of
hard to see how it could work, nor did Google help me find one.

NPSF is "Dryseal USA (American) Standard Fuel Internal Straight Pipe
Thread" according to _Machinery's Handbook_.

On Mon, 14 Jan 2008 05:56:18 +1300, Tom
wrote:

Stealth Pilot wrote:

On Fri, 11 Jan 2008 21:28:50 -0600, "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 1/4 x 18 straight thread is NPSF (straight) DRYSEAL ...whatever that
means in english. it is a pipe thread.

in a taper that thread would be NPTF (taper) DRYSEAL.


Stealth Pilot

I hope you read you pilot notes with greater concentration than you did the
original post? :-)

Tom

why what's the problem? I read the details from my mullins thread wall
chart. it lists the 18 pitch threads exactly as quoted.

the post answers the last two sentences of the question.
a. what's it used for? - pipe thread.
b. his suspicion that it was a mistake is probably wrong since there
is a listed application for the thread size.

pilot notes .....pilot notes??? now what are they? a description of
the machining standards for slocumbe drills?

Stealth :-) Pilot

"Stealth Pilot" wrote in message
...
On Mon, 14 Jan 2008 05:56:18 +1300, Tom
wrote:

Stealth Pilot wrote:

On Fri, 11 Jan 2008 21:28:50 -0600, "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 1/4 x 18 straight thread is NPSF (straight) DRYSEAL ...whatever that
means in english. it is a pipe thread.

in a taper that thread would be NPTF (taper) DRYSEAL.


Stealth Pilot

I hope you read you pilot notes with greater concentration than you did
the
original post? :-)

Tom

why what's the problem? I read the details from my mullins thread wall
chart. it lists the 18 pitch threads exactly as quoted.

the post answers the last two sentences of the question.
a. what's it used for? - pipe thread.
b. his suspicion that it was a mistake is probably wrong since there
is a listed application for the thread size.

pilot notes .....pilot notes??? now what are they? a description of
the machining standards for slocumbe drills?

Stealth :-) Pilot

You missed the fact that this thread in question is 1/4" diameter. It
is *not* a pipe thread. It is a non-standard 1/4" tap.

Harold

On 2008-01-14, Stealth Pilot wrote:
On Mon, 14 Jan 2008 05:56:18 +1300, Tom
wrote:

Stealth Pilot wrote:

[ ... ]

a 1/4 x 18 straight thread is NPSF (straight) DRYSEAL ...whatever that
means in english. it is a pipe thread.

in a taper that thread would be NPTF (taper) DRYSEAL.


Stealth Pilot

I hope you read you pilot notes with greater concentration than you did the
original post? :-)

[ ... ]

why what's the problem? I read the details from my mullins thread wall
chart. it lists the 18 pitch threads exactly as quoted.

Out of curiosity -- what does the mullins thread wall chart say
is the OD of the thread. IIRC, the original question said that the
actual measured OD was 1/4", while any pipe thread would normally be
named for something closer to the bore size, thus requiring a much
larger diameter for the threaded part.

the post answers the last two sentences of the question.
a. what's it used for? - pipe thread.

Not clear -- until you can tell us that your chart shows the OD
as 1/4". It could easily be some other thread with the same pitch, but
a smaller OD, for a different function.

b. his suspicion that it was a mistake is probably wrong since there
is a listed application for the thread size.

Again -- are you sure about the OD?

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On 15 Jan 2008 04:58:17 GMT, "DoN. Nichols"
wrote:

On 2008-01-14, Stealth Pilot wrote:
On Mon, 14 Jan 2008 05:56:18 +1300, Tom
wrote:

Stealth Pilot wrote:

[ ... ]

a 1/4 x 18 straight thread is NPSF (straight) DRYSEAL ...whatever that
means in english. it is a pipe thread.

in a taper that thread would be NPTF (taper) DRYSEAL.


Stealth Pilot

I hope you read you pilot notes with greater concentration than you did the
original post? :-)

[ ... ]

why what's the problem? I read the details from my mullins thread wall
chart. it lists the 18 pitch threads exactly as quoted.

Out of curiosity -- what does the mullins thread wall chart say
is the OD of the thread. IIRC, the original question said that the
actual measured OD was 1/4", while any pipe thread would normally be
named for something closer to the bore size, thus requiring a much
larger diameter for the threaded part.

the post answers the last two sentences of the question.
a. what's it used for? - pipe thread.

Not clear -- until you can tell us that your chart shows the OD
as 1/4". It could easily be some other thread with the same pitch, but
a smaller OD, for a different function.

b. his suspicion that it was a mistake is probably wrong since there
is a listed application for the thread size.

Again -- are you sure about the OD?

Enjoy,
DoN.

I wonder what that glow is around here?
11mm tapping drill for both.

oh I get it. the light's on.

Stealth Pilot

Stealth Pilot wrote:
On 15 Jan 2008 04:58:17 GMT, "DoN. Nichols"
wrote:


On 2008-01-14, Stealth Pilot wrote:

On Mon, 14 Jan 2008 05:56:18 +1300, Tom
wrote:


Stealth Pilot wrote:

[ ... ]


a 1/4 x 18 straight thread is NPSF (straight) DRYSEAL ...whatever that
means in english. it is a pipe thread.

in a taper that thread would be NPTF (taper) DRYSEAL.


Stealth Pilot


I hope you read you pilot notes with greater concentration than you did the
original post? :-)

[ ... ]


why what's the problem? I read the details from my mullins thread wall
chart. it lists the 18 pitch threads exactly as quoted.

Out of curiosity -- what does the mullins thread wall chart say
is the OD of the thread. IIRC, the original question said that the
actual measured OD was 1/4", while any pipe thread would normally be
named for something closer to the bore size, thus requiring a much
larger diameter for the threaded part.


the post answers the last two sentences of the question.
a. what's it used for? - pipe thread.

Not clear -- until you can tell us that your chart shows the OD
as 1/4". It could easily be some other thread with the same pitch, but
a smaller OD, for a different function.


b. his suspicion that it was a mistake is probably wrong since there
is a listed application for the thread size.

Again -- are you sure about the OD?

Enjoy,
DoN.


I wonder what that glow is around here?
11mm tapping drill for both.

oh I get it. the light's on.

Stealth Pilot

What is it about the following that you are unable of grasp?:
From the OP:

"I know that is the designation for a standard NPT tap, but
this tap is actually 1/4" in diameter."

Tom

On 2008-01-15, Stealth Pilot wrote:
On 15 Jan 2008 04:58:17 GMT, "DoN. Nichols"
wrote:

On 2008-01-14, Stealth Pilot wrote:

[ ... ]

why what's the problem? I read the details from my mullins thread wall
chart. it lists the 18 pitch threads exactly as quoted.

Out of curiosity -- what does the mullins thread wall chart say
is the OD of the thread. IIRC, the original question said that the
actual measured OD was 1/4", while any pipe thread would normally be
named for something closer to the bore size, thus requiring a much
larger diameter for the threaded part.

the post answers the last two sentences of the question.
a. what's it used for? - pipe thread.

Not clear -- until you can tell us that your chart shows the OD
as 1/4". It could easily be some other thread with the same pitch, but
a smaller OD, for a different function.

b. his suspicion that it was a mistake is probably wrong since there
is a listed application for the thread size.

Again -- are you sure about the OD?

[ ... ]

I wonder what that glow is around here?
11mm tapping drill for both.

11mm divided by 25.4 mm per inch gives a diameter of 0.4331",
and the actual tap diameter would have to be a bit over the tap drill
size, while the original poster said that *his* tap was measured at 1/4"
(that is 0.250" so these are *very* different taps.

oh I get it. the light's on.

Where? Not in your head, yet, apparently.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---