# Are NPT & NPS close enough to work?



## strantor (Sep 30, 2014)

I've got 2 tanks on my HPU; a diesel tank and a hydraulic fluid tank. Both have 3/4"NPT bungs on them, and I want to install tank level transmitters on them. The only affordable thing I can find are these sensors which screw into a 3/4"NPS. I know the two NP variants have the same pitch and TPI; I believe the only difference is one is tapered and one isn't. I just want to be sure; if I order these sensors, _*will they screw into my tanks*_? I figure they won't make any kind of a seal, but this isn't a pressured connection, and I don't really care about having/not having a pressure seal.


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## JimDawson (Sep 30, 2014)

It's been my experience that it will work fine.  The non-tapered thread will screw into the tapered fitting because the thread is the about same size as the small end of the male tapered thread.


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## John Hasler (Sep 30, 2014)

They are not considered interchangeable.  In practice that plastic NPS adapter should screw into a NPT hole ok.  However, I see no reason you couldn't replace the adapter they supply with your own.  Call them and ask them about it.


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## 12bolts (Oct 1, 2014)

JimDawson said:


> .....the thread is the same size as the small end of the tapered thread.


I disagree Jim. The smaller diameter of a tapered fitting is marginally less than the minor diameter of a comparable sized straight thread. However i agree that the the fitting will thread together. I would use a good dollop of PTFE goop as a sealant also.

Cheers Phil


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## JimDawson (Oct 1, 2014)

JimDawson said:


> .......same size as the small end of the tapered thread.





12bolts said:


> I disagree Jim. The smaller diameter of a tapered fitting is marginally less than the minor diameter of a comparable sized straight thread. However i agree that the the fitting will thread together. I would use a good dollop of PTFE goop as a sealant also.
> 
> Cheers Phil



Phil, you are of course correct, I was trying to say what you said, but didn't do it very well.  What I meant to say was ..._.about same size as the small end of the *male* tapered thread._  I need to quit trying to do 3 things at once.


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## mzayd3 (Oct 1, 2014)

I'll add my two cents. &nbsp;I am an electrician. &nbsp;Part of my job is process instrumentation and controls. &nbsp;We routinely run into this problem. &nbsp;All electrical fittings with a male thread (aside from threaded pipe ends) use straight threads. &nbsp;All fittings with a female thread use tapered threads. &nbsp;As far as instrumentation (what you are dealing with), most level transmitters use straight threads. &nbsp;They work just fine in tapered threads and even hold pressure when properly sealed. &nbsp;Go with it!


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## Falcon67 (Oct 1, 2014)

I sell fittings that adapt the ports in a Ford C4 to -AN braided lines.  The trans ports are NPS and I re-thread 1/8-27 NPT fittings to work.  The seal to the trans is by o-ring and not thread.  Also, threading a NPT fitting into the trans may cause the fitting boss to split.  Normal line pressure is around 30 psi but I have tested a hand tightened NPS/o-ring seal to 100 PSI.  

YMMV as they say, but I would not use a NPS thread in an NPT port.  You may only be able to seal the connection with JB Weld.  My NPS tap passed over maybe the first 2-3 threads on a fitting, then begins cutting.  If there is NO pressure and some bit of sealer, it MAY hold for some period of time.


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## 12bolts (Oct 1, 2014)

The OP's choice of level gauge is designed for screwing into the top of a drum. He is not looking for a pressure/leak tight seal. It will screw in to the tank he has and perform its intended function.

cheers Phil


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