Seacock, Nps or Npt?

kito · Jan 7, 2016

I just went through this myself. Thru hull fitting threads are NPS (straight). Ball valves are npt (tapered) both ends. I replaced all my gate valves with flanged seacocks (Buck Algonquin). The seacocks have a nps outlet to thread onto the thru hull fitting and a npt inlet. Maybe you can unscrew those old gate valves and reuse your thru hull fittings. I had to cut mine off....too corroded.

centerline · Jan 7, 2016

the NPT stands for National Pipe Taper.... the NPS stands for National Pipe Straight.
the npt is a taper fit, so the more you thread it on, the tighter it gets... whereas the nps is a straight cut thread similar to what a bolt has and will not seal when used with NPT fittings... they will fit together, but will not tighten properly and is prone to leaking....
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Capt jgw · Jan 7, 2016

A straight thread male going into a tapered thread female will only screw in a couple threads deep. Not only prone to leaking, but very easy to break off. Don't do it. Straight to straight and tapered to tapered only. That's why a hardware store ball valve won't work. You should use a real marine seacock. I try to use the cheaper alternatives when I can, but not here.

Charles Erwin · Jan 7, 2016

Revue the information here:

http://www.pbase.com/mainecruising/seacock_primer

MaineSail is the author.

By the way - don't hesitate to contribute something to keep his information going - it is reliable and you will probably not find this kind of information elsewhere.

Charles

Gunni · Jan 7, 2016

I would not attempt to reuse a 35 year old thru hull fitting. You can mate a Groco "hybrid thread" thru-hull with a groco ball valve. See more here.

Franklin · Jan 7, 2016

All this talk of nps not fitting with npt is not all that accurate. Hunter and many boat makers do this as it is the most common available (nps thru hulls with npt seacocks -- yes real bronze marine seacocks). Some of mine have lasted 20 years. I just did one myself a few weeks ago. It screwed on all the way to almost the end (about 8 threads) where the tapering along with the tape tightened it down real good.

If you put about 5 wraps of Teflon tape it will work just fine. After all, we are only talking about 15 psi.

Now granted, going with flanged with both npt is best but to say anything else will leak is not correct. Mine never leak.

kito · Jan 7, 2016

I replaced all my thru hulls with new bronze Groco or Algonquin fittings and flanged seacocks. Like CJ said.....below waterline fittings is not the place to save a buck. I followed MS's directions except I used g10 for the backing plate and didn't go through the hull with the 3 mounting bolts. I used flat head silicone bronze screws c'sunk on the opposire site and bedded it in Bondo Glass.

heritage · Jan 7, 2016

I followed Gunni's link to the Groco site and found this to be an excellent tutorial on seacocks: http://www.groco.net/svc-main.htm. You have to click on the picture of the seacocks and then Tech Bulletin.

jibes138 · Jan 11, 2016

If you go with the valve you already have (Groco ball valve) then you should install the adapter flange that MaineSail shows on his website. This will allow a NPT to NPT connection.

jibes138 · Jan 12, 2016

toliver said:
Dave, thank you for pointing out that thread adapter. WM sells it for $40...I will try to buy one and take with me on soon-to-install project.
Wonder if I could install without a backing plate?

Your photo shows there is already a backing plate in the location but it is painted over so the material isn't identifiable. You may find it difficult to remove the existing valve from the thru hull fitting due to age and hardened sealant or corrosion. In replacing mine I used an oscillating tool and just cut it off, only takes about a minute to cut through a thru hull fitting. If you can get your valve off then just thread on the adapter flange with fresh sealant and drill three holes for the screws (use bronze screws, lock washers, and nuts) and then epoxy over the heads on the outside of the boat after properly countersinking them. Remember, parts and tools are cheap compared to yard labor so doing it yourself you are saving a ton of cash but do it right by following MaineSail's instructions.

BigEasy · Jan 12, 2016

When I removed some of my thru hull fittings, I grinded them on the outside of the hull. The hull mating surface of a mushroom thru hull fitting is not flat from the outer edge of the flange to the inner thru hull portion of the fitting. I surmise that is designed in this manner so that there is a sufficient thickness of bedding sealant that forms a water tight seal. If it were perfectly flat, all of the sealer would be compressed and extruded from beneath the flange. This design makes it quick and easy to grind it off. Just grind the flange where the tubular thru hull portion protrudes thru the hull until grinding to the sealant bed; the entire flange will then be detached from the tubular portion. With a chisel, putty knife, or screw driver you can separate the flange from the hull; the flange will appear with "donut hole" in the middle. On the inside of the hull, grab the remainder of the thru hull still attached to the seacock or ball valve with vise grip pliers and pull the fitting out of the hull.
This technique was quite simple and quick; about 5 minutes per thru hull fitting. I used a grinding disc on my Roto Zip tool; an angle grinder would work well also.

jibes138 · Jan 12, 2016

I cut the thru hull to remove the ball valve, then I cut both sides of the nut and removed that. The remaining piece of the thru hull should be easy to knock out, you can put a pipe wrench on the threaded part remaining and turn it a few times to break it loose from the sealant, then it should come right out. A new thru hull is only 20 bucks or so for a 3/4 so saving the old one is up to you depending on how much time you want to spend on it. In my case I couldn't budge the valve so I just cut it off, If your valve comes off easily you should be all set, you will still need to remove the nut if you use the flanged adapter. If it was me I'd want to remove the thru hull and clean it up and the boat surfaces and use new sealant to reinstall it. The three screws are to prevent it from rotating when you put torque on it turning the valve handle. If you break the thru hull sealant by rotating the thru hull in service you will have a leak, a sinking boat, and will need a haul out to fix it again. MaineSail shows an alternative method to tap the backing plate and put studs in that so you don't drill thru the hull. I drilled thru the hull but his method should be fine but you will need to replace your backing plate while you are at it to get material you can tap like G10. This is not hard but takes a little time while you let epoxy cure and so forth. Just print out MaineSails directions and follow them to a T

njlarry · Jan 13, 2016

Big Easy has a great way to remove them with a grinder. You can also use a long blade on a reciprocating sawzall from the outside, or a metal blade on an oscillating saw from the inside or outside. Any scraches will be under the mushroom head. I have used both G10 and plywood as backer plates without any issues. The builder of my boat use plywood backers that all lasted 30+ years.
Maine Sail shows the correct Bristol method that I have used but according to GROCO the ball valve can be attached directly to the mushroom threads if less than 1 1/2 inches are exposed between the thru hull nut and edge of the ball valve housing. Saving the old nut, you can thread it on the new mushroom to cut the threads perfectly.

kito · Feb 1, 2016

What is the hose clamp for?

Seacock, Nps or Npt?

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