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Compass Marine How To | all galleries >> Compass Marine How To Articles >> Seacock & Thru-Hull Primer/Pre Information > So What is a Combination Thread Thru-Hull
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So What is a Combination Thread Thru-Hull
28-JAN-2009

So What is a Combination Thread Thru-Hull

There is such a thing as a "combination thread" thru-hull. A combination thread is a half arsed way of making a ball valve thread onto a standard straight threaded thru-hull. Essentially the tips or peaks of the threads are machined off so the ball valve can thread over them and get more thread purchase.


While this is clearly better than using a regular straight threaded thru-hull with a ball valve it is still a "shortcut". One major draw back is that you can not shorten the thru-hull to minimize interior height. The longer the stem inside the hull the more leverage there will be to potentially snap it off. If you MUST use a thru-hull with a ball valve threaded on please source some combination threaded thru-hull fittings.


If you click on the picture to make it larger you'll clearly see how the first few threads are shaved down. This does NOT make it an NPT thread just a band aid approach to get some more thread contact area. This design was likely driven by large production boat builders in an attempt to save a few pennies. While certainly better than mixing NPS to NPT I would think twice before using a combination threaded thru-hull with a ball valve.

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Roland Lloyd 05-Aug-2011 20:09
I own a Tartan sailboat and want to replace the head water inlet parts. The existing system is a is a bronze flush face fitting with a 1/2 inch NPS straight thread, an old gate valve all attached to a length 3/4 Inch ID hose and on to the head itself. Here is the problem: I have a new 1/2 flush mount fitting (tappered 45 degrees) to the the hull surface and it is exacttly 2.4 inches OD (like the old fitting) on outside face. Groco only makes this in 3/4 ID flush mount thru hull but the OD is only 2 inches outside. They also do not make the triangular shaped adapter that I used in all of the largwe thru hulls on the boat. I like the adapter concept withthe NPS to NPSM feature. I would like to use the ftting(I bought the 2.5 OD thru hull in a marina parts store, sale item) but I do not know the mfgr. Who else carries the adapter?

TKS
rollo