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
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
1/4 - 18 is a NPS (National Pipe Straight) standard size.
-- Ed Huntress
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.
Some thermocouple fittings are 18tpi and fit a 1/4" hole.
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
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.
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
But not in this case.
Harold
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
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
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
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