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holding tank fittings

Discussion in 'Technical Discussion' started by Brian, May 4, 2011.

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  1. Brian, May 4, 2011
    #1
    Brian

    Brian Senior Member

    Joined:
    Nov 14, 2005
    Messages:
    378
    Location:
    Halfmoon Bay, B.C. Canada
    Forgive me for this boat help in this yacht forum but I have run into a snag now and hoping for some help from some of you.

    We are installing a new polyethylene holding tank in our little Albin cruiser.
    The tank has "spin fittings" that are bonded in. The "in & out's"
    are 1 1/2" and the vent is 3/4" and are standard tapered pipe thread.
    The fittings I have bought are Marelon male pipe thread to hose
    adaptors...http://www.forespar.com/products/boat-marine-plumbing-
    elbow.shtml
    Being "pipe thread", I thought these fittings would be tapered as
    well, and work with the tank inserts but they're not. I have tried them in the tank with teflon tape but they keep threading into the tank and don't tighten like I thought they would.
    I guess these are not the correct fittings to use?
    What type do I need?
    Help!

    Thanks

    Brian
  2. dennismc, May 4, 2011
    #2
    dennismc

    dennismc Senior Member

    Joined:
    Jan 29, 2009
    Messages:
    1,177
    Location:
    Vancouver BC
    holding tanks

    That's odd, I changed 3 gate valves to female threaded marelon and they threaded on real tight with no problems, are you sure the tank fittings are pipe thread for sure ?? hopefully not metric !!!!
  3. Bill106, May 4, 2011
    #3
    Bill106

    Bill106 Senior Member

    Joined:
    Jan 9, 2010
    Messages:
    390
    Location:
    Beaufort NC
    The Marelon fittings you have are designed as tailpiece fittings for sea cocks and are indeed straight thread so they will continue to screw in until they bottom the threads. Even with generous amounts of tape or 5200 they will not seal like you will want them to on a holding tank and probably leak odors at best. Most hardware stores carry nylon hose barbs, they are cheaper and will work just fine in this application. (Save those Marelon ones for seacock replacements)
  4. Brian, May 4, 2011
    #4
    Brian

    Brian Senior Member

    Joined:
    Nov 14, 2005
    Messages:
    378
    Location:
    Halfmoon Bay, B.C. Canada
    Hi dennismc,
    Thanks for the quick reply!
    I have called the tank manufacturers and they say they're standard NPT.
    The polyethylene is a soft material compared to the Marelon, which makes me wonder if it will just force the tank material "out".
    I can wind in a 1 1/2" MPT/hose 90 Marelon elbow into the tank by hand all the way in and doesn't tighten.
    This article from Forespar makes me wonder...
    http://www.forespar.com/pdf/techTips/M6-Marelon-Understanding-Threadforms.pdf
    Especially... "A male threaded Marelon® fitting will not work in a female tapered thread".
    Well, what does one use?
    They mention their HC sieries fittings but they only go up to 1 1/4"
    I appreciate any help, thanks.
    Brian