3/4" NPT thread on mixing valve

[Florida Gator]

If you look at the attached picture, in the background you can see a mixing valve I installed about 8 years ago. I soldered the connections. I am remodeling my bathroom and replacing it with the new one in the foreground of the picture. It calls for a 3/4" NPT fitting. Every one I have tried screws about 1/2 way into the opening (as you can see in the picture) It doesn't feel right when screwing it in. I have looked everywhere and everything I have found is like this. Also, what is the best way to go from the 3/4" fitting to a 1/2" copper pipe?
Thanks!


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[Reach4]

It is normal for NPT to not screw in all of the way.

When screwing it in for real, I would use both good PTFE tape and pipe dope such as Rectorseal #5. Then apply a lot of torque with a wrench. I tighten as much as I can, and come back a half hour later and tighten some more. I am not a plumber.

In the case of a nipple, I would concentrate on applying the torque to whatever goes into the other end of the nipple. When you tighten that, holding the other end, both sides of the nipple get tightened up at the same time.

 

[Sponsor]

 

[Florida Gator]

If that is the case, then I should be able to use a fitting like this one:


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And I would solder this to the copper pipe before screwing it in with the tape (so I am not heating up where the tape is)

 

[Reach4]

And I would solder this to the copper pipe before screwing it in with the tape (so I am not heating up where the tape is)

Yes.

 

[Dgeist]

+1 on the pipe dope. I had a heck of a time getting the service kit on my tankless water heater installed. It had the same issue. Finally, I switched to a better PTFE product and got bigger wrenches and was able to get almost a full additional turn on the fitting. A quick lookup in a mechanical engineering reference says for brass and steel, 54 in/lb should be your target. That translates to finger tight + 2-3 full turns with teflon lube.

BTW, don't worry too much about the tape once the joint is torqued. It's purpose is to get the threads tight enough against each other to seal, not to make a seal itself. If the only thing making a liquid seal is the dope or tape, the joint is probably going to leak eventually.

 

[hj]

NO THREAD is perfect. One function of any sealant is to fill the miniscule gaps in the threads.

 

[Florida Gator]

Thanks. I was hoping that to be the case, but wasn't feeling confident with a thread that only screws half-way.

 

[Dgeist]

NO THREAD is perfect. One function of any sealant is to fill the miniscule gaps in the threads.

Agreed. That and the minor deformation of the thread surfaces at torque make the seal solid. My point is don't DEPEND on the dope/tape to seal it for you. It's a supplement to the properly mated threads, not a replacement.

 

[Jadnashua]

Tapered pipe threads are an interference fit...they are slightly different in the socket verses the male end...they are designed to get tight as you tighten them up (it's sort of like driving in a wedge), and to fill the gaps, you need pipe dope or tape (or both). Unlike something like a hose which relies on a gasket at the end, a pipe thread doesn't seal without the dope or tape, but does tighten up without being bottomed out.

 

[Terry]

My dad used a bar of soap to lube up the threads in the old days. Both sides of the joint are tapered, and they can seal threaded in that way. I prefer dope, tape or both on the joints though.
And yes, they almost have never bottomed out for me. If they do it kind of worries me.

 

[Jadnashua]

If the tap and die are both sharp and not worn out, and, used properly, it can seal without anything, but in today's market, to be sure, use pipe dope and/or tape on any tapered pipe thread. Some of the threaded connections are just crappy.

 

[MACPLUMB]

Get a 3/4" cmip X 1/2" cop adp. thread in tight "Without" tpe or dope, then solder around the threads

 

[Bgard]

If the tap and die are both sharp and not worn out, and, used properly, it can seal without anything, but in today's market, to be sure, use pipe dope and/or tape on any tapered pipe thread. Some of the threaded connections are just crappy.

Made in China comes to mind

 

[Florida Gator]

Get a 3/4" cmip X 1/2" cop adp. thread in tight "Without" tpe or dope, then solder around the threads

You're saying I could get a copper threaded male fitting, screw it into the brass (I think it is brass?) mixing valve. And then just solder the threaded connection?

 

[Dgeist]

You CAN solder pipe threaded fittings, but since sweating pipe is basically having the liquid metal chase a fluxing agent to form a seal, it's not going to work nearly as well (basically you're asking the solder to follow an irregularly heated spiral many times the distance it would be in a sweat fitting. If you have threads, connect the threads the right way and they won't leak. If you have a sweat fitting, solder it right and it won't leak. You can always get a mixing valve with sweat fittings. Most plumbing supply places provide those, but they're less common at the big boxes.

 

[MACPLUMB]

You're saying I could get a copper threaded male fitting, screw it into the brass (I think it is brass?) mixing valve. And then just solder the threaded connection?

YES ! have done it many times the solder follows caprillo action around the treads and seal just like a regular solder joint