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Thread: Connecting straignt threads (NPSC) to tappered (NPT)

  1. #1
    Junior Member parkfront's Avatar
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    Default Connecting straignt threads (NPSC) to tappered (NPT)

    Great site!! I'm a builder working on my own bathroom addition and need some help with a shower valve installation.

    I am in the process of laying out the location of the supply line connections for an external Meynell thermostatic shower mixing valve, and was wondering what the best method to connect 3/4" straight thread inside the wall. I got this setup (see attached photos) from a remodel years ago and want to use it in my new tiled shower. It is a Meynell Victoria safemix thermostatic valve.

    Because the valve body will be installed after tiling, I will have to be able to rotate the inlet elbows in order to fit the valve between them (they each have a lip that protudes inside the body about 1/8"). Which means I can't solder them in their final position. I'm not sure if that would be recommended anyway. Because the unsoldered connection will not be accessible (exterior wall), I want a connection with the lowest potential for leaking.

    I was considering to either :
    1. thread it into a 3/4" NPT (tapered thread) with teflon tape and pipe dope.
    2. or find a straight threaded connection with a washer.

    Any sugestions would be greatly appreciated. I would also love to find a link to an owners manual and spare parts for this valve. Meynell is a British company who's valve business was sold to Mira/Kohler, who are also in England I believe.

    Thank you
    Gregory
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  2. #2
    In the Trades Bob NH's Avatar
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    The direction you are going is asking for serious trouble.

    You are trying to install the valve in a manner and in a place where it shouldn't be installed. It is designed to be installed in a place where you have access to the back side.

    The large straight threads are for a clamping nut to hold the valve in place. You may be able to make a connection at the end by using one of the flexible connectors that are often used as a toilet supply. That kind of supply should not be use in an inaccessible space.

    You might find a supply hose in a "big box" store. If unsuccessful there, try a plumbing supply house. But if you hide that kind of connection behind tile it will not conform to any codes and you are risking grief when one fails.

  3. #3

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    If you hook it up with PEX, you can use a PEX swivel adapter. It has a plastic seal rather than depending on the threads to do the sealing.



    I have some old ones like this that have a polybutylene ring, but I think the newer ones have a vinyl seal.

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    Plumbing Contractor srdenny's Avatar
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    How about a couple of these?
    They can be found at:
    http://www.sealconusa.com/strainreli...t_3425_br.html
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  5. #5

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    Quote Originally Posted by srdenny
    How about a couple of these?
    Metric wouldn't be helpful.

  6. #6
    Plumbing Contractor srdenny's Avatar
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    Cut them to the desired length, then put a tapered thread on them. If you can't do it, a machinist can. BTW: The reason I suggested the metric adapters is that I've used them in a similar situation and they worked. Only drawback is the enlarged hole needed to accept the female hex head of the adapter.

  7. #7
    Junior Member parkfront's Avatar
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    Thank you for your responses.


    I still need a little guidence in what approach to take. I understand that a Pex or "washered" connection needs to be accessible. I'm assuming (based on shower head connections) that NPT connections are commonplace and legal in inaccessible walls? Has anyone had success adding a tappered thread on the end of a straight thread?

    Has anyone installed one of these and made it secure and code worthy...in an inaccesible wall.

    Any watertight good looking ideas on making it accessible from inside the shower?

    Thanks again
    Gregory
    Last edited by parkfront; 01-28-2008 at 03:32 PM. Reason: change question

  8. #8
    Junior Member parkfront's Avatar
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    srdenny,

    Did you use that adaptor on a metric male thread or one like mine? I know this has 14 TPI but I'm not sure on the thread angle because it was made in England and may be a 55 degree instead of 60 degrees. Not sure if that would matter anyway.

    G

  9. #9
    Plumbing Contractor srdenny's Avatar
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    I've taped and doped straight thread fittings made in England (Perrin and Rowe, Barber Wilsons) and screwed them into fip fittings and not had leaks. However, I'd prefer tapered to tapered. Like I said, any machine shop worth a hoot would be able to put a taper on the end of those nipples. It would cost, but you wouldn't have to touch the wall.

  10. #10
    Junior Member parkfront's Avatar
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    Thanks srdenny!

    I took your advice and went to the machine shop at S.F. City College and did the taper myself with an adjustable die set. I can thread it into a female NPT fitting 3-1/2 turns by hand, so I figure with some pipe dope wrapped 4 times with teflon tape should seal it up good.

    I will have to use a NPT extender (a brass NPT to NPT version of the metric fitting you suggested above) connection in order to reach the already set rough NPT supply lines. Is there a good method to permanantly bond the extender to the rough fittings, so I don't have to worry about disturbing that seal when I install the valve and the newly threaded elbows?

    See image below

    Thanks again for your help everyone
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  11. #11
    Plumber patrick88's Avatar
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    I assume those chrome nuts swivel. That is how I have installed similar valves. If not I would install the 90's on the valve. Then pass the nipples through the wall and connect with a brass union.
    I'm just starting to work with an old friend of mine to bring solar electric and hot water systems, wind turbines, Flex Fuel Boilers, batteries, hydroponic gardening, books, pellet grills and more. Also the parts for DIY installation.

  12. #12
    Junior Member parkfront's Avatar
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    yes they do swivel. What kind of brass fitting would you use if they had to go in the wall with the valve attached?

    Any favorite pipe dope you use? I have Rectorseal 5, but the otherday someone sugested a different one called something like " two and one".I will do a search and possibly start a new thread.

    G

  13. #13
    Retired Defense Industry Engineer jadnashua's Avatar
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    It's easier on one project to keep one thread...the history helps people understand what's going on and why. If it doesn't get responses, just add something to pop it back to the top.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  14. #14
    Plumbing Contractor srdenny's Avatar
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    "Is there a good method to permanantly bond the extender to the rough fittings, so I don't have to worry about disturbing that seal when I install the valve and the newly threaded elbows?"
    Connect the nipple extension to the nipple and screw them into the wall. They'll both tighten as you turn the nipple. Knowing when to stop is a tough one to explain. Under tighten-leak. Over tighten-cracked brass fitting. Good Luck.

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