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Old 01-01-2011, 15:21   #1
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NPS vs NPT Thru Hulls

I need to replace a couple of slightly leaking thru hulls on my 1984 Oday 28. The configeration currently in the boat looks to be a thru hull mounted on plywood backing plate (now rotten) then a bronze ball valve attached directly to the thru hull. I'ver been reading up on the prefered NPS thru hull with a true flanged seacock also with internal NPS threads, but don't quite understand the preference for NPS threads as these are not a mechanical seal as are NPT threads. The set up I have was used extensively for many years and I understand the advantages of the flange adding strength and the matching threads and the need for the thru hulls to be furnished as NPS as they might need to be cut shorter, but I'm struggling with a couple of things. First the fact that I don't really want to add several more holes in my hull. In addition it seems that NPT threads would provide a better long term seal. I've looked for the so called combination thread thru hulls, but can't find them anywhere. (I know Groco will make them on special order). In addition my 17 year old ball valves are working just fine. I have looked at the flanged Groco adaptors and see there is a method of attaching to the backing plate, but again I'm not too sold on NPS threads. I've read everything I can find, but can someone please explain why NPS would provide a good seal as oposed to NPT. BY the way I played around with some seacocks and thru hulls at West the other day and the NPS to NPS seem loose while NPT to the NPS seems to make up nicely.

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Old 01-01-2011, 15:33   #2
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I went with the NPS thru-hulls to NPT ball valves and have no regrets. Not as strong as the more expensive stuff, but I don't plan to hit them with a sledge hammer. Loctite makes a very expensive goop to seal the NPS-NPS threads, but you can use teflon tape with the NPS-NPT joints.

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Old 01-01-2011, 15:44   #3
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I have excellent results using the flange adapter from groco.

Thru hull to flanged adapter to ball valve. Sealed with 4200 and holds up extremely well. I first glued the wood backing plate to the hull with 4200, then put the hardware together. I used short lag screws into the wood backing plate but chose not to bolt thru the hull.

Working well
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Old 02-01-2011, 06:56   #4
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NPS is a straight thread and the NPT is a tapered thread. I also used the groco flange and it is great. Fits the thru hull and the valve great. Now I can just replace a bad ball valve with out messing with the Thru hull. When I replaced the thru hulls I found that the ball valves were held on bu just two turns.If I had bumped it at all or hit a floating log, I know the valve would fail.
You don't have to drill a new hole, just use the old hole. You don't want to sink for lack of a $40 part.
That derelict boat was another dream for somebody else, don't let it be your nightmare and a waste of your life.
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Old 02-01-2011, 08:36   #5
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ABYC H-27, Through-Hull Fittings and Drain Plugs: “Threads used in seacock installations shall be compatible (e.g., NPT to NPT, NPS to NPS).”

This entry specifically prohibits the mixing of incompatible thread types, was added to the standard in 2008.

See Main Sail’s excellent Seacock & Thru-Hull Primer (photo-essay)
Seacock & Thru-Hull Primer/Pre Information Photo Gallery by Compass Marine at
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