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Loose 1/4" and 3/8" NPT Threaded Holes

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  • Loose 1/4" and 3/8" NPT Threaded Holes

    I've run across 2 NPT threaded holes that are loose around the male threads that thread into them. Well, not loose but they don't go all the way snug before the male threads bottom against their hex (one is a 1/4NPT to AN-6 adapter, the second is 3/8" NPT a sensor). Neither side is easily replaceable. One in particular (the 1/4"NPT) is in a tight spot so it would be quite difficult to drill out and even more difficult to use a 3/8-1/4"NPT bushing. They are threaded into a coolant passage.

    In both cases I just need them to seal, and I've been debating what I could use on the threads to seal them, and still be able to pull them out in the future (preferably not easily, but something short of drilling them out also). Something like epoxy is a last ditch answer, I don't want to fight them that much to pull them out if I have to. I've contacted both Permatex and Loctite for suggestions and neither has responded.

    I've considered things like Permatex #1, their various aviation sealants, Indian Head Shellac adhesive, Anaerobic gasket maker http://www.permatex.com/products-2/p...t-maker-detail , the assorted thread sealants rated for straight threads into coolant passages (this is the stuff that i've been using recently http://www.henkelna.com/product-sear...=8797903224833).

    Any suggestions what will work?

  • #2
    Are new male fittings replacing ones installed previously?

    NPT is tapered. If this is a replacement situation, then
    perhaps the replacements have a longer tapered section
    and thus bottoming where their shorter predecessors
    could tighten without bottoming.

    Regardless, can the outer ends of the present male fittings
    be shortened/relieved such that full mechanical thread
    engagement occurs ?

    A wrap of pipe tape or daub of pipe sealant still counts on
    mechanical engagement for sealing under pressure. My vote
    is that in the absence of that mechanical thread engagement,
    sealing will be illusionary.

    Edit: Oops. On rereading, I see that bottoming occurs between
    the hex shoulder of the male fitting against the surface of the
    female threaded hole. Not at the male tip against an interior wall.

    Sorry.

    Still do not think a sealant is your answer for excessive gap-o-sis.

    .
    Last edited by EddyCurr; 08-10-2015, 09:47 PM.

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    • #3
      No chance that this is an instance of mixing BSPT
      (British Standard Pipe Thread) and NPT fittings ?

      .

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      • #4
        Perhaps you could remove some material from the shoulder, so the threads can engage deeper.

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        • #5
          JB weld the loose fittings together
          Hobbyist: someone who makes something sound harder than it is.
          Professional: someone who cant afford to be a hobbyist.

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          • #6
            Before gooping up the threads, I 'd try 2 or 3 wraps of Teflon tape and see how that works out.
            Location: Long Island, N.Y.

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            • #7
              Originally posted by RichR View Post
              Before gooping up the threads, I 'd try 2 or 3 wraps of Teflon tape and see how that works out.
              +1.

              Teflon tape would be my first choice/try as well.

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              • #8
                Is it possible that you're dealing with Dryseal threads instead of NPT?If all else fails Loctite 567 thread sealant would be my pick.
                I just need one more tool,just one!

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                • #9
                  I would try the teflon tape first and if that did not work I would move up to a product called Xpando http://www.xpando.com/. This is a black powder that is mixed with water to form a paste. The paste is applied to the threads and then assembled. The paste will dry out and expand slightly as it drys and will both bond and seal. I used this product when I worked as a pipefitter for a refrigeration company. We used this on black iron pipe thread fittings in ammonia refrigeration systems. Never a leak. It is possible to unscrew the fittings after it drys, but it is really tough.

                  Robin
                  Robin

                  Happily working on my second million Gave up on the first

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                  • #10
                    NPT details

                    https://www.google.com.au/search?q=g...t+thread+chart

                    https://www.google.com.au/search?q=n...IVR56mCh0HRgHX

                    Comment


                    • #11
                      Originally posted by EddyCurr View Post
                      Are new male fittings replacing ones installed previously?
                      new

                      Still do not think a sealant is your answer for excessive gap-o-sis.
                      I agree, but I don't really have the option of replacing the parts that are "overthreaded," and they'd be pretty tough to weld up and re-cut, at least a whole ton of work.

                      Originally posted by EddyCurr View Post
                      No chance that this is an instance of mixing BSPT
                      (British Standard Pipe Thread) and NPT fittings ?
                      Nope, recognize both... anyway, I don't know of any standard carparts that use BSPT... Honestly I think I've only seen BSPT once and it wasn't on a car

                      Originally posted by doctordoctor View Post
                      JB weld the loose fittings together
                      Last option... I know that as soon as I epoxy them together the fitting will get munged and need to be replaced or the sensor will die, and then I have a bigger problem to deal with.

                      Originally posted by oldtiffie View Post
                      +1.

                      Teflon tape would be my first choice/try as well.
                      RichR and oldtiffie, I was under the impression that thread tape (and teflon pipe dope) were both mostly there to lubricate the tapered threads so you can thread them together tighter, that they don't really do much sealing. That was the major reason why I discounted them from the get go. The only reason I considered still using a thread sealant intended for use with head bolts that thread into a coolant passage was that in that case it's something designed to seal a thread that will never tighten against itself (straight cut bolt threads), so there is some chance of that working... maybe I'm wrong here. It's probably the easiest thing to try, but I _really_ don't want to take this thing back apart and fix it if it doesn't work.

                      Comment


                      • #12
                        Could they actually be NPT straight and you use one of the o-ring style fittings?

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                        • #13
                          Originally posted by Silverback
                          Originally posted by EddyCurr
                          No chance that this is an instance of mixing BSPT
                          (British Standard Pipe Thread) and NPT fittings ?
                          Nope, recognize both... anyway, I don't know of any standard
                          carparts that use BSPT... Honestly I think I've only seen BSPT
                          once and it wasn't on a car
                          Bit of a tease, aren't you. Oh, well. I'll play along ...

                          So these are cooling system fittings where the female parts
                          belong to a non-North American part, because they are not
                          NPT. Not British, either - that would be too obvious. Not
                          German, those would be DIN.

                          Could we perhaps be dealing with something of Japanese mfr ?

                          I struggle with the notion that your female cooling system
                          fittings are NPT items that have worn to the extent that
                          they now allow a new male NPT fitting to pass through so
                          far that the male's hex bottoms against the outside lip of
                          the female boss.

                          .

                          Comment


                          • #14
                            Cursory research shows that numerous Japanese auto
                            mfrs employ JIS / ISO / BSPT imperial-dimensioned
                            tapered threads. Including:
                            • Datsun
                            • Isuzu
                            • Lexus
                            • Nissan
                            • Subaru
                            • Toyota


                            Any number of topics can be found on websites where
                            owners are grappling with swaps, gauge installations
                            and so on.

                            Point of note about TPI differences between BSPT & NPT
                            • 1/8": BSPT = 28 TPI / NPT = 27 TPI
                            • 1/4": BSPT = 19 TPI / NPT = 18 TPI
                            • 3/8": BSPT = 19 TPI / NPT = 18 TPI


                            .

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                            • #15
                              Originally posted by Silverback View Post
                              RichR and oldtiffie, I was under the impression that thread tape (and teflon pipe dope) were both mostly there to lubricate the tapered threads so you can thread them together tighter, that they don't really do much sealing. That was the major reason why I discounted them from the get go. The only reason I considered still using a thread sealant intended for use with head bolts that thread into a coolant passage was that in that case it's something designed to seal a thread that will never tighten against itself (straight cut bolt threads), so there is some chance of that working... maybe I'm wrong here. It's probably the easiest thing to try, but I _really_ don't want to take this thing back apart and fix it if it doesn't work.
                              A taped part will be much easier to remove if it doesn't work (both now and in the future, think repairs) as opposed to a sealant that sets up.
                              If there is enough material, cutting back the shoulders on the two items so they can thread deeper may be an option. You mentioned one of
                              the items is a sensor, so a fixture might be in order. Chuck up a piece of steel in the lathe and drill and tap it so your device can screw in without
                              bottoming out. Use a parting tool to shave 10 to 20 thousands at a clip off the shoulder to remove however much you think you need to. Then
                              see if the part snugs up without bottoming out.
                              Location: Long Island, N.Y.

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