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Old 03-12-2013, 06:11   #1
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NPT Fittings, How tight should they be

All of my bronze seacocks are fitted with PVC tail pieces and I will be replacing them with Groco bronze in the next few weeks. I'm unclear how tight to fit the threads. I'm hoping for a bit of leeway in requirements for this so I can get the nozzles pointed in the direction best for fitting the hoses. I will be using pipe dope.

The seacocks and the tailpieces have cast hex nuts. Any hints about the tools you use, how to tighten these in the enclosed area and how to make sure you don't break anything in the process would be very useful too.
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Old 03-12-2013, 18:36   #2
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Re: NPT Fittings, How tight should they be

Just went through this with 12 of them.

If you are going through the trouble of replacing the thru hulls you might as well as use the Groco with the backing plates. Forget about the bronze rings. Use these:

GROCO MARINE PRODUCTS

As far as how tight to get them... I actually tightened mine too much and broke off the little keyways on the inside of a few before I realized I was going about it all wrong.

My issue is that there is no place on a hull that is actually flat. Either the outside of the hull is curved or the inside is curved and also not actually flat. Probably both. In the end I created molds using west system and the groco thru hulls. I could get away with this because my hull is over 1.25 inches thick in places. Otherwise you will need to use a backing shim glassed in.

The point is... if the surfaces are properly mated you don't need to tighten a lot. Otherwise you can tighten all you want and you will still not get a good mating.
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Old 03-12-2013, 19:59   #3
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Re: NPT Fittings, How tight should they be

In terms of the actual tightening of NPT fittings, they really don't have to be very tight at all. "Not very tight" is of course a subjective term but to put it practically, a NPT joint made using quality components only has to be tighten a smig past hand tight. The subtle interlocking axial taper of the male and female halves of the joint creates enormous locking friction within the joint. A couple of wraps of Teflon tape will act both as a lubricant and sealer, and is less mess that pipe dope. Chain-grips or a chain tourniquet work really well for tightening joints in confined spaces.
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Old 03-12-2013, 20:15   #4
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Re: NPT Fittings, How tight should they be

I suspect it is a preference, when it comes to Teflon pipe tape I would never use or recommend your basic white. Instead I would suggest the yellow for your standard piping. An insight is this, in many states it is Illegal to use the white on any gas piping because of so many issues with Leaks.
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Old 03-12-2013, 20:50   #5
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Re: NPT Fittings, How tight should they be

My experience with NPT fittings has taught me that the torque required varies with the material, the diameter & the quality of the threads.

I have seen PVC joints fail after several months in service from being assembled too tight. After time the female fitting split. I have never seen bronze fittings split after assembly, even when very tight, unless water froze inside of them.

Naturally, a 2" NPT fitting will need a lot more torque than a 1/4" NPT fitting.

There are different flavors of NPT. There is standard NPT, NPT level seal, NPT rail, etc. If you try to mate two different versions, you will have difficulties.

If you are using name brand bronze fittings in standard NPT, then I would start at around 25 foot pounds for a 1" fitting & check for leaks. If leaks are present, then I'd go a little tighter until things seal up. If you get much past 50 foot pounds & still have trouble, then maybe your dope wasn't very good or else your thread quality was poor.

I have generally had good luck with plain white Teflon tape. I have also had good luck with non-hardening paste type pipe-dope from Permaseal. I know some people who recommend using both tape & paste at the same time, but I haven't tried that myself. When wrapping the tape, be sure to wrap it on the male in the same direction that the female will screw onto it. I start wrapping at the large end of the male & then work towards the small end when using tape that is more narrow than the thread width.

With good quality brass NPT fittings & white Teflon tape, I have been able to get good seals on every gas I have tried that had a molecule larger than Helium. I don't have experience piping fuel gasses.
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Old 03-12-2013, 20:54   #6
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Re: NPT Fittings, How tight should they be

White Teflon tape is not propane or gas resistant. Works fine on water however. The yellow tape is for gas and required for all gas fittings in house construction. Personally like the teflon pipe dope way better than the tape. So much easier to work with and suitable for gas as well as water.
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Old 03-12-2013, 20:55   #7
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I love the groco flange adapter.
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Old 03-12-2013, 21:55   #8
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Re: NPT Fittings, How tight should they be

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I love the groco flange adapter.
Yes... they are very solid and its nice knowing that you can just switch out the valves at some point in the future.

Which brings up another point... those groco valves have marelon seals inside, so be sure to grease them up well with marelon grease when installing. You can't add too much.
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Old 04-12-2013, 01:08   #9
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Thanks for the replies. Very helpful. I'm not actually replacing the thruhulls/seacocks just the tailpieces. The thruhulls are Buck Algonquin and I don't have it in the budget to replace any unless they are damaged.
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Old 04-12-2013, 11:30   #10
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Re: NPT Fittings, How tight should they be

I usually tighten NPT threads by hand as tight as I can then use a wrench to tighten up to 1 or 1-1/2 turns more. This allows some lee way to get the fitting pointing in the right direction. You'll develop a feel for it.

My favorite sealant is Leak Lock. You can seal pipes carrying just about anything you are likely to find on a boat, including gasoline and propane.

Link: Leak Lock by Highside INC.
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Old 04-12-2013, 12:07   #11
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Re: NPT Fittings, How tight should they be

Several of the seacocks on our boat were fitted with the same PVC crap. We also replaced with groco. See the excerpt from the NPT guide I used for tightening. It indicates number of turns past finger tight. Sorry for the poor image but I don't know how to share PDF files.

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Old 04-12-2013, 12:14   #12
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Re: NPT Fittings, How tight should they be

To confirm NPT sizes.

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Old 09-12-2013, 22:05   #13
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Re: NPT Fittings, How tight should they be

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Several of the seacocks on our boat were fitted with the same PVC crap. We also replaced with groco. See the excerpt from the NPT guide I used for tightening. It indicates number of turns past finger tight. Sorry for the poor image but I don't know how to share PDF files.

That's an interesting chart. Do you know if those torque spec's are for brass/bronze fittings? Or might they be for black iron pipe?
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Old 10-12-2013, 07:00   #14
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Re: NPT Fittings, How tight should they be

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That's an interesting chart. Do you know if those torque spec's are for brass/bronze fittings? Or might they be for black iron pipe?
The guide indicates that it is for all "metal to metal" NPT fittings. I down loaded it from Surplus Center
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Old 11-12-2013, 08:04   #15
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Re: NPT Fittings, How tight should they be

I find it unusual that a single torque spec would be given for all metal to metal NPT fittings. It has been my experience that black iron pipe will take much more torque than brass/bronze of the same size.

Perhaps it is best to heed the first sentence of the last paragraph that was included with the torque table.

Thank you for the reference,
Jim
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