NPT to NPS through hulls

[Roger Long]

It's not the treads, it's the flange.

I decided to fix the remaining NPT/NPS miscogony below the waterline by replacing the head intake line thru hull. (Here, I'll try one of those big pictures.)

I was able to get this smaller thru hull (It turned out to be 3/4 not 1/2 as posted above) out easily and having it, the Groco adapter, and the ball valve all together gave me another chance to look at the thread issue. Here's what I found:

1) The thru hull was surprisingly loose in the adapter. I could make it click until the end was hard up against the bottom. Since it has to be cut a bit short to be sure of fully taking up, this is a joint that will never be tight without tape or compound.

2) The NPS thru hull went fully into the ball valve and bottomed out. It could be wiggled a bit when backed off two turns.

3) The 3/4 NPT spigot on the adapter went only about half of the thread length into the ball valve with about the torque that could be applied in the field. This is only 3 - 4 threads. It could also be wiggled when backed off two turns but not quite as much as the NPS thread.

4) Just to be sure this wasn't a mismatch with my original 1980 thru hull, I took the parts into the store and tried a new Groco thru hull. The results were identical.

Conclusions:

1) The physical strength of the NPS into the NPT is probably greater since nearly twice as many threads are engaged and the end of the male fitting bears on the bottom of the hole.

2) The fluid tightness of the NPS into NPT is certainly less due to more tolerance between the metal surfaces. This might be an issue at water service pressures but at the heads that thru hulls experience should be well within the capability of the pipe dope to seal.

3) These findings are a function of the relatively short thread length of the ball valves. They would not apply to full length tapers of couplings and other fittings.

4) Your NPT ball valve is not going to break off of your NPS thru hull.

BUT, big big but:

This is a frequently used valve. Even though it isn't unusually stiff, I could clearly see the effects of the strain and working of the small thru hull nut on the plywood backing block. A few more years and water getting under the nut would have broken down the wood and the backing block would have started rotting. With just normal valve forces, the strain at this point is clearly unacceptable. NPT ball valves on NPS thru hulls should be replaced for this reason alone.

 

[Maine Sail]

Here's what I found:

1) The thru hull was surprisingly loose in the adapter. I could make it click until the end was hard up against the bottom. Since it has to be cut a bit short to be sure of fully taking up, this is a joint that will never be tight without tape or compound.

NPS to NPS are not sealing threads like NPT to NPT. The threads of the thru-hull and especially the area around the head of the mushroom make the seal when marine sealant is applied. There is also a seal between the flanged adapter and the backing plate.

2) The NPS thru hull went fully into the ball valve and bottomed out. It could be wiggled a bit when backed off two turns.

Goes back to what I said about thread machining tolerances. Some are cut heavy and some cut light. Sounds like you had two that fit really loosely. Sometimes they are very tight.

3) The 3/4 NPT spigot on the adapter went only about half of the thread length into the ball valve with about the torque that could be applied in the field. This is only 3 - 4 threads. It could also be wiggled when backed off two turns but not quite as much as the NPS thread.

Sometimes happens see above comment but you have tapered matching threads designed to fit together and a better seal for water tightness.

4) Just to be sure this wasn't a mismatch with my original 1980 thru hull, I took the parts into the store and tried a new Groco thru hull. The results were identical.

My guess is that's lucky. My Groco valves thread onto my adapters leaving only a few threads showing if that. The depth or lengths of NPT threads are supposed to be standardized and valves rarely if ever bottom out all the way onto the male threaded device. Each batch can be slightly off due to tap and die wear..

Note how many threads are still showing on the right and the depth of the nipple in the coupling:

I just found this picture from when I was installing mine. You can see how deep my valve went onto the adapter by the remains of sealant. I pre-installed the valve and tightened it with pipe dope to get a total height measurement but I removed it after I made that measurement to make more working room:

As I said machining plays a big role from batch to batch 7 brand to brand..

Conclusions:

1) The physical strength of the NPS into the NPT is probably greater since nearly twice as many threads are engaged and the end of the male fitting bears on the bottom of the hole.

This will not always be the case and can just as easily go the other way of too few threads engaged.

2) The fluid tightness of the NPS into NPT is certainly less due to more tolerance between the metal surfaces. This might be an issue at water service pressures but at the heads that thru hulls experience should be well within the capability of the pipe dope to seal.

In a hydronic heating system running 30 PSI or a steam system running 100+ PSI there is no way it would seal but on a boat they can be sealed and remain dry. Thousand of boats are leak proof with NPS to NPT but they are under very, very low pressures. It's still an improper thread match and one that is not advised by the manufacturers or the ABYC.

3) These findings are a function of the relatively short thread length of the ball valves. They would not apply to full length tapers of couplings and other fittings.

Yep those ball valves don't have very deep threads especially the smaller sizes but they are still strong. I've seen valves like this in industrial applications running 500 PSI..

4) Your NPT ball valve is not going to break off of your NPS thru hull.

Nope that's not likely at all but it could become loose and start to leak and as in my situation something could slide into the ball valve and crack or snap the thru-hull. It happened to me and scared the living bejesus out of me. Mine did not snap all the way off but it did fracture and was spraying water in through the crack in the threads.

BUT, big big but:

This is a frequently used valve. Even though it isn't unusually stiff, I could clearly see the effects of the strain and working of the small thru hull not on the plywood backing block. A few more years and water getting under the nut would have broken down the wood and the backing block would have started rotting. With just normal valve forces, the strain at this point is clearly unacceptable. NPT ball valves on NPS thru hulls should be replaced for this reason alone.

Couldn't agree more...

Nice job on the seacocks!!

 

[Hermit Scott]

I wondered what the little bolt was for on the flange of the grocos. I see Roger has used them to bond his through hulls to each other. Now I know what they are for.
On the subject of galvanic corrosion. I took apart my groco 750 water strainer and it had a stainless basket in a bronze frame. So I have heavy pitting in the bronze I have to sand out. Who is designing these things?
By the way where are you guys getting your bronze nuts and bolts?

 

[Maine Sail]

By the way where are you guys getting your bronze nuts and bolts?

Hamilton Marine or Jamestown Distributors stocks them.

Hamilton has a great nut and bolt aisle.. From the ladder all the way to the end wall, floor to ceiling, is stainless, bronze, galvy, copper & brass fasteners..... Just a tad better than the selection at the local West Marine.

 

[sailingdog]

understating it just a bit there. prices are far better too.

Just a tad better than the selection at the local West Marine.

 

[Roger Long]

Just a tad better than the selection at the local West Marine.

And even cheaper, with "If they don't have it, they don't make it selection", is Fastenal. Super search function on their website.

http://www.fastenal.com

Portland, (Maine) has two stores. Lisa, manager of the one on Maine Street (Route 1) in South Portland is a sailor so service is especially good when she finds out the fasteners are going into a boat.

I buy a lot of fasteners at Hamilton though out of convienience and loyalty. If I need less than a dozen, it isn't worth the time and gas.

 

[Maine Sail]

K.L. Jack

And even cheaper, with "If they don't have it, they don't make it selection", is Fastenal. Super search function on their website.

http://www.fastenal.com

Portland, (Maine) has two stores. Lisa, manager of the one on Maine Street (Route 1) in South Portland is a sailor so service is especially good when she finds out the fasteners are going into a boat.

I buy a lot of fasteners at Hamilton though out of convienience and loyalty. If I need less than a dozen, it isn't worth the time and gas.

And..... even cheaper (on some items) is our local nut and bolt supplier K.L. Jack!! I buy most of my nuts and bolts by the box. If I ever need a 1/4 X 20 nyloc nut I just buy a box of 100 for about $3.00. That's about 4 nylocs at WM so I figure I get 96 for free.

Hamilton has much better prices when you have an account but K.L. Jack (LINK) is even cheaper.. Fastenal is great too and it's good to know about Lisa! I have not found their silicon bronze selection very good at the Forest Ave. location though..

I have three of these.. They have paid for themselves in "wasted time trips" about 20 times over..

 

[Hermit Scott]

I deal with Fastenal all the time here and they give me really good pricing. I think I need 1/4"-20 2 1/2" long pan head bolts for the flange adapters. What do I need to specifically ask for for the material type? Can I just ask for bronze or will I end up with something cheap that is incompatible with the seacocks?

 

[Maine Sail]

I deal with Fastenal all the time here and they give me really good pricing. I think I need 1/4"-20 2 1/2" long pan head bolts for the flange adapters. What do I need to specifically ask for for the material type? Can I just ask for bronze or will I end up with something cheap that is incompatible with the seacocks?

Silicon bronze fasteners is what they are generally referred to as.