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Old 11-12-2013, 07:57   #16
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Re: NPT Fittings, How tight should they be

Do you guys actually have a pipe wrench that measures torque?
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Old 11-12-2013, 08:25   #17
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Re: NPT Fittings, How tight should they be

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Originally Posted by pbiJim View Post
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
Agreed, I would never torque a NPT fitting, but having zero experience, I really liked having a "turns past finger tight" guide to get me in the ballpark. I could then adjust based on leaking or to position the fitting.
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Old 12-12-2013, 17:40   #18
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Re: NPT Fittings, How tight should they be

SeaSloth - If I understand you correctly you're replacing some PVC elbow tail pieces with Groco elbow tailpieces.

There have been many excellent responses, but here's another response to consider.

1. Over torquing can cause galling of the metal fitting materials, not to mention cracking them. Plastics will just expand or crack.

2. It is best to use non-hardening pipe paste with metal NPT fittings. The issue is the sharp metal threads can cut the tape. Although given the low water pressures the vast majority of below the waterline fittings will encounter (5 psi at most) this should not be much of an issue, but I'd still go with the paste with either metal or plastic.

2. The ASTM standard for NPT threads is 4.83 threads hand-tight to closure. I.E. all the thread surfaces locking. However, that's the ideal, I've tested NPT marine fittings and, hand-tight they range from 8 turns and still loose (Shields barbed fittings) and 2 turns to hand tight (West Marine white nylon fittings). Even the high quality threaded hose fittings I had molded were + or - 10% of that number, but at 5 turns with just a few wraps of tape would hold 50 psi.

3. THE GROCO tailpiece is well made and should be close to the 4.83 turn number hand tight.

4. I'd use non-hardening pipe seal and hand tighten the elbow, then go 3/4 to 1 or so turns more to position the elbow to where it fits best. Unless the fitting is improperly machined, which I doubt, you'll be leak free.
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Old 12-12-2013, 17:54   #19
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Re: NPT Fittings, How tight should they be

I faced a similar issue, installing NPT elbow fittings in my water pump">raw water pump. There is only one angle they will work at, but of course this is not where the optimum torque is reached. Now I use teflon paste on both threads, then teflon tape on the male thread. This seems to work a treat, not one single leak since I started doing this, and it allows you some leeway in adjusting the NPT fitting.
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Old 14-12-2013, 19:20   #20
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Just a little tighter than snug, or you will strip the threads.
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Old 14-12-2013, 19:22   #21
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Agreed mark Teflon paste works well.
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Old 03-02-2014, 05:04   #22
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Re: NPT Fittings, How tight should they be

Now that I have done a few of these I have discovered a few things.
1. Teflon paste works well. The boat is still out of the water but when I fill the hoses full to get a good 2 to 3 feet column of water over the connection I see zero leaks
2. Once I get the fitting hand tight it is very difficult to get a single full turn with the wrench. Ultimately I did what was suggested...tightened by hand and then cranked it around to point where I wanted. Ultimately, I didn't have much choice. They had to be at least hand tight and had to point in a certain direction.
3. The hard part wasn't actually with the fittings themselves but with the hoses. Getting the old hoses out and new ones in was a real PITA.
4. Old head hoses are rank.

Thank you all for your help. The job is almost done.


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Old 03-02-2014, 06:24   #23
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Re: NPT Fittings, How tight should they be

Lot of thread sealant tighten hand tight then use a wrench to align to the hose. Always works for me
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Old 07-02-2014, 17:08   #24
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Re: NPT Fittings, How tight should they be

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3. The hard part wasn't actually with the fittings themselves but with the hoses. Getting the old hoses out and new ones in was a real PITA.
Special hose pulling tools exist. They look like a screw driver that has been tapered to a dull point & bent a little near the end.
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