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Npt fitting torque? #5329470 01/22/20 11:47 AM
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motor_oil_madman Online Content OP
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I've had some bad luck lately on the boat breaking off stainless screws, so I'm a bit skittish with npt fittings. One thing is it's hard to judge the tightness in most cases because there isn't much room to work and I can only get maybe 1/4-1/2 turn at a time. I changed out a fitting that someone else put in and they didn't have any threads showing. (first time I've seen that) anyways it's a brass fitting going into aluminum so I'm not going to get carried away. When I put in the new 1/4" npt fitting I had about 3 threads showing and it really wasnt wasnt any tighter than the one with all the threads in. I felt it was going to be too tight to make it one more full turn to get it back in the correct position. Opinions? It's doesn't seem like it's going to go anywhere, moving it by hand. Should I just check for leaks? Or give it another turn?


2007.5 dodge cummins 6.7 liter. Chevron Delo400 15w40. 7000 mile or 250-300hr intervals.

Re: Npt fitting torque? [Re: motor_oil_madman] #5329476 01/22/20 11:53 AM
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Imp4 Offline
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If you're unsure and concerned about breakage, pressure test the system first and check for leakage as long as it's not a safety issue.

With respect to the broken stainless fasteners, there's a lot of junk out there. Also, small fasteners require small torques to work properly, and there are different torque values for different fastener grades. Do your homework and find out what is the correct fastener for your application and the correct torque for that fastener.

I'll bet you reduce your broken bolt incidents by 90%.


2001 Mitsubishi Galant 2.4 - 165k+
Grandma's creampuff...
Re: Npt fitting torque? [Re: motor_oil_madman] #5329506 01/22/20 12:27 PM
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motor_oil_madman Online Content OP
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Well the broken screws are a result of not drilling a big enough pilot hole. It's just amazing that I can still break them putting them in by hand with a screw driver. Stainless is soft compared to a regular wood screw that you can drill all the way through the board and not break.


2007.5 dodge cummins 6.7 liter. Chevron Delo400 15w40. 7000 mile or 250-300hr intervals.

Re: Npt fitting torque? [Re: motor_oil_madman] #5329510 01/22/20 12:30 PM
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fasteners vary for sure + most stainless ones are lo torque + of course galling is worse with stainless. a search will arm you with some important tips

Re: Npt fitting torque? [Re: motor_oil_madman] #5329523 01/22/20 12:43 PM
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But here there are two completely different situations. A typical fastener is loaded in shear or tension whereas the NPT connection is relying on an interference fit of the tapered threads to provide a gas or liquid-tight connection. Generally an NPT connection is not made for structural integrity.

That NPT connection needs to be tight enough to not leak and not a lot more. You run the risk of cracking the pipe or the fitting by making it too tight, just as you can do on a water pipe. Brass going into aluminum won't have to be Cro-Magnon tight.to seal.


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Re: Npt fitting torque? [Re: kschachn] #5329548 01/22/20 01:30 PM
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Imp4 Offline
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Originally Posted by kschachn
But here there are two completely different situations. A typical fastener is loaded in shear or tension whereas the NPT connection is relying on an interference fit of the tapered threads to provide a gas or liquid-tight connection. Generally an NPT connection is not made for structural integrity.

That NPT connection needs to be tight enough to not leak and not a lot more. You run the risk of cracking the pipe or the fitting by making it too tight, just as you can do on a water pipe. Brass going into aluminum won't have to be Cro-Magnon tight.to seal.

Everything in this post is spot on.
Well said.


2001 Mitsubishi Galant 2.4 - 165k+
Grandma's creampuff...
Re: Npt fitting torque? [Re: motor_oil_madman] #5329565 01/22/20 01:50 PM
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Won't there be an issue of electrolysis joining dissimilar metals like brass and aluminum? I would think aluminum might be a good anode and the brass would be a cathode.


2020 Nissan Rogue SL AWD times two! (white and black)
Re: Npt fitting torque? [Re: Oily_Thing] #5329650 01/22/20 03:49 PM
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Imp4 Offline
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Originally Posted by Oily_Thing
Won't there be an issue of electrolysis joining dissimilar metals like brass and aluminum? I would think aluminum might be a good anode and the brass would be a cathode.

Reading up a bit on galvanic corrosion seems to bolster your concern.
Maybe OP should seek a competent professional mechanic before attempting this repair. Being out on the water is a terrible place to discover an error.


2001 Mitsubishi Galant 2.4 - 165k+
Grandma's creampuff...
Re: Npt fitting torque? [Re: motor_oil_madman] #5329705 01/22/20 05:07 PM
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hallstevenson Online Content
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As others have mentioned, generally with NPT, if it's not leaking, it's "tight". I've never seen, and I don't think it exists, typical torque values for NPT. I used to design centrifugal pumps and parts and we used NPT fittings extensively.

Re: Npt fitting torque? [Re: motor_oil_madman] #5329890 01/22/20 08:58 PM
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During my 40 years in the gas service industry there were many opinions heard as to the correct tightening of black iron pipe fittings. Some liked to tighten to having two threads showing as long the joint felt tight which is of course subjective. Many gas controls have brass into aluminum fittings which seem to work well. Worked with a man who came from the sprinkler industry who would use permatex compound and buried the threads on iron pipe. After the sealant setup it was almost impossible to break the threads loose usually crushing the pipe rendering it useless. He used to like saying there will be no leaks in his work. Don`t know why there were no split fittings.


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Re: Npt fitting torque? [Re: motor_oil_madman] #5329948 01/22/20 10:41 PM
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There are tables showing how far a particular size nipple is to screw into a fitting. Search the web. That data is shown in Machinery's Handbook as well. Use the original Rectorseal pipe dope if you don't want something to leak. Good luck unscrewing it. Rectorseal now markets a Teflon paste in a tube. The original Rectorseal is a yellow pipe dope.

I highly recommend Machinery's Handbook. A wealth of information. Every gear head should have one.

Re: Npt fitting torque? [Re: motor_oil_madman] #5329972 01/23/20 12:14 AM
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I noticed that too. The liquid sealers seem to somewhat glue the fitting in place once it cures. Not necessarily locking it in place though, just securing it.


2007.5 dodge cummins 6.7 liter. Chevron Delo400 15w40. 7000 mile or 250-300hr intervals.

Re: Npt fitting torque? [Re: Imp4] #5329975 01/23/20 12:17 AM
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Originally Posted by Imp4
Originally Posted by Oily_Thing
Won't there be an issue of electrolysis joining dissimilar metals like brass and aluminum? I would think aluminum might be a good anode and the brass would be a cathode.

Reading up a bit on galvanic corrosion seems to bolster your concern.
Maybe OP should seek a competent professional mechanic before attempting this repair. Being out on the water is a terrible place to discover an error.



I love how asking about something automatically deems me as incompetent.


2007.5 dodge cummins 6.7 liter. Chevron Delo400 15w40. 7000 mile or 250-300hr intervals.

Re: Npt fitting torque? [Re: Imp4] #5330084 01/23/20 08:01 AM
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Originally Posted by Imp4
If you're unsure and concerned about breakage, pressure test the system first and check for leakage as long as it's not a safety issue.


Yes , tighten it until you think it is good & then pressure test it .

Remember NPT has a taper thread , which gets tighter , as you turn it . Not like a bolt / screw with no taper to the thread .


Wyr
God bless
Re: Npt fitting torque? [Re: motor_oil_madman] #5330537 01/23/20 05:57 PM
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Imp4 Offline
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Originally Posted by motor_oil_madman
Originally Posted by Imp4
Maybe OP should seek a competent professional mechanic before attempting this repair. Being out on the water is a terrible place to discover an error.

I love how asking about something automatically deems me as incompetent.

Asking about something doesn't deem you as incompetent.

Breaking down out on the water after others have suggested that another set of eyes look at a problem certainly demonstrates incompetence though...

Good luck!!!
Stay safe.
cheers


2001 Mitsubishi Galant 2.4 - 165k+
Grandma's creampuff...
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