Alittle help with NPT,NPSI Comparison

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Cadillac

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So I have a really big job coming up from a pipe fitter friends company. They need 38 blind flanges drilled and tapped with a 1/2npt test port. This is how they cap the ends of pipe and then pressure test their work.
So anticipating the job I ordered some tooling. While scouring flebay I found 3
1/2-14 npsi taps with a shank so I could Chuck it in a chuck for 9 dollars. Well I had bought them not looking at the fine print and being excited I found what I needed for a steal. Well receiving them I noticed the npsi not npt.
Doing alittle investigating I see npsi it is a straight pipe thread. Minimal taper. Says that you use sealant and for lower pressures. Npt doesn’t require sealant to seal and is for higher pressures. Big question is can I run the straight taper and then follow up with a npt to taper the hole. Letting the straight taps do the bulk of the work. I have 3 1/2-14 Npt taps already one with a interrupted cut which are suppose to be the catsass, two regular.
If not I’ll get back to looking for some. I just want the threads to spec and if I can use what I already have well that’s a plus. If not it was worth the try. Any experience on the matter would be appreciated thanks.
 
I don't see why you couldn't do that as long as there is still enough meat to follow with the npt tap
Npt joints still need some sealant in my limited experience
-Mark
 
IIRC, NPTF threads can be assembled dry as they create a mechanical seal. NPT threads require the use of sealant of some kind; Teflon tape, or paste.
 
I think the straight pipe tap is larger than the taper tap, or maybe about the same pitch diameter as the big end of the taper tap; you should be able to look up the respective pitch diameters in Machinery's Handbook. Bottom line, I think the proposed plan will not work out.
 
NPT threads are depth sensitive.
 
First thanks for the quick responses guys. I shouldn’t have said don’t require sealant you are correct. Doing alittle more searching I’ve read you can use one for the other but not the other way around. I have some time so I’ll keep looking for some npt taps with a shank. I’m gonna try a test piece and see how far I can use the straight tap it has a lot of lead in on it.
Yes I am aware of the depth my plan was to finish the last couple threads by hand.
I did some ruff measurements and the measurement on the nps od. Is .826 and that gets me about 4 threads from start on a npt. Npt major is .836 so there is alittle room gonna have to give it a try. I’ll keep yahs updated. Thanks


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NPT taps are usually driven in until 3 - 5 threads are still showing. It is doubtful that you could drive in a taper pipe tap held in a drill chuck, it takes a fair amount of torque, especially when the tap is near full depth, this is why pipe taps have squared shanks, they are generally driven with a special tap driver when machine tapping. The ones that I had were with a morse taper shank with a square hole in the end.
 
If it was a "big job" for my "pipe fitters friend's company", I would want to do the job properly.
To do it properly you'll need a proper taper reamer to prepare the hole(s), a proper NPT tap(s) and a proper gage to prove the product is within spec.

If you don't choose to tool-up, why not farm out the NPT to a shop who is? Pretty basic stuff.
 
.The starting pitch diameter of a 1/2 NPT external thread is .7758" The pitch diameter of a 1/2 NPS is .7718" - .7769"
 
I have never used a taper reamer to prepare for tapping with a NPT tap but it would probably reduce the torque required to tap the hole. As far as "within spec", given the use the parts are are to be given, doing the 3-5 threads showing would be good enough.
 
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