Thread size deckfill fitting

I switched out my water tank on my 1981 Marine Trader trawler. The original fiberglass tank failed.
The deck pipe is a 1 1/2 inch ID threaded pipe. A 1 1/2 inch hose barb screws into the deck fitting from inside the lazzerete, where the water tank is located. I’ve tried NPT, NPS. To no avail. The thread looks finer than NP. Taking the fitting out to properly measure could be destructive.
The fitting is the ubiquitos deck fill marked water. Any ideas what the thread type is? The original deck fill was lost.

Greetings,
Mr. gd. Very good chance the thread is metric. Thread pitch gauge is less than $10 at Princess Auto OR borrow from a friend. Just don't drop through the fitting.

The standard I believe is 1 1/4 , 11 1/2 NPSM Straight but there are a couple other sizes out there. Any boat junk yards around you?

I’ve never seen one that wasn’t NPT. I assume there is a reason you can’t reuse the old hose barb.

I have run into a lot of npt fittings that would only go a couple of threads before jamming. Is this what’s happening?

If it is really not an npt thread, it might be easier to just change the deck pipe.

I had a hard time matching up the caps after I busted a vinyl cap and water fouled my stbd fuel tank. I found one online but it took a couple buy/returns before I finally found the right set.

I think RTF's gauge, if appropriately sized, would be worth the cost.

grahamdouglass said:

I switched out my water tank on my 1981 Marine Trader trawler. The original fiberglass tank failed.
The deck pipe is a 1 1/2 inch ID threaded pipe. A 1 1/2 inch hose barb screws into the deck fitting from inside the lazzerete, where the water tank is located. I’ve tried NPT, NPS. To no avail. The thread looks finer than NP. Taking the fitting out to properly measure could be destructive.
The fitting is the ubiquitos deck fill marked water. Any ideas what the thread type is? The original deck fill was lost.

Click to expand...

I'm a bit confused...most deck fittings have barbs ONTO which the hose goes...no fitting that threads into the barb. However, I think I know why nothing fits: Y'all are correct that all deck fitting are NTP. But all those made before sometime in the mid--'80s were 16 TPI (at least the cap was, so I'm assuming that would also be true for any threads that extend through the barb end)...everything since then is 11 TPI. You're not gonna find any 16 TPI fittings today...I learned that when I lost the cap to the pumpout fitting on my 1979 Trojan F32. The only solution is a new deck fill.



--Peggie

The cap threads vary by manufacture, some are straight pipe threads and some are what ever the manufacture wanted to use.

Some deck fills come with a built in hose barb and some are threaded on the bottom to accept a screw in pipe fitting or hose barb. I’ve seen hundreds of these things from a bunch of manufacturers and they all had tapered pipe threads.

That’s just my experience in the USA. European manufactures may use another thread standard.

The caps on my original bronze deck fills are not any of the standard threads in use today, I think a common situation for 1970’s Taiwan built trawlers.

This concerned me also since if I loose one I have no easy way to find a replacement, and pump-out threaded adapters won’t fit the waste deck fitting. Even the caps for the fuel, water and waste seem to be slightly different and none of the caps have retainers.

As the prior poster said you’ll need to get lucky with a marine recycler on finding a cap or replace the deck fill entirely.

HopCar said:

The cap threads vary by manufacture, some are straight pipe threads and some are what ever the manufacture wanted to use.



Some deck fills come with a built in hose barb and some are threaded on the bottom to accept a screw in pipe fitting or hose barb. I’ve seen hundreds of these things from a bunch of manufacturers and they all had tapered pipe threads.



That’s just my experience in the USA. European manufactures may use another thread standard.

Click to expand...

The most common thread in my neck of the woods is 1-1/2” BSP 11 TPI . I suspect in the U.S. NPT would be more likely and 1-1/2” NPT is 11.5 TPI so although very close they are not compatible. To confuse things further the NPT thread is loosely referred to as 11 being the closest whole number to 11.5. As previously mentioned closely measure the number of threads with a thread gauge but make sure the gauge you buy has both 11 TPI and 11.5 TPI. You may find that you have a mixture of BSP and NPT. Compounding the issue is that both are available in tapper and parallel and the thread angles also differ. Peggy’s advise to buy them as a matched set could prove to be the easiest way as it’s all fine and well to identify the threads but it doesn’t solve the problem, it just identifies what the problem is.

Maybe you could simply use a rubber hose and a clamp.
My water tank, and fuel tanks, have bronze threaded pipe screwed into the tanks. The other end has a hose shoved right onto the bare pipe Other end goes to water or fuel fitting with the typical looking barb appearance. Mine have never leaked. I have double clamps on the fuel fill pipe. All the barb (bump at bottom of fitting) does is make it real hard to pull it off when a clamp is on the hose. Does not improve the seal. Is anyone going to be pulling real hard on the hose? NO.

On the pipes to hose, I have a good 2 inches of rubber hose over the pipes..

If all you have is thread to match to a hose, you can put some sealer T plus 2 Rector seal etc... non hardening for water on it, but I bet you the clamp will seal the rubber hose to pipe and or threaded pipe ends fine. Big advantage of the Rectorseal, keeps the rubber from gluing itself onto pipes.

I would expect all deck fill caps to be a parallel thread as they seal on the o-ring rather than the thread. That means it would likely be National Pipe Thread Parallel (NPTP) if made in USA or British Standard Pipe Parallel (BSPP) if made in Europe.
BSP threads are generally much finer threads in sizes over 1.5 inch.

I have encountered at least one Asian manufacturer of deck fittings that are 1-1/2", 14 tpi instead of the more common 11.5 tpi. It's absolutely maddening because you can't get any adapters that fit for pump outs, etc.


After trying a variety of expensive, custom machined adapters, I realized it would be much easier and less expensive to replace the deck fittings with ones that have a more common thread. Sea Dog makes a line of very nice polished stainless deck fittings, and they are only about $50 a pop. Problem solved.


One of the things in my bag for my first trip to Taiwan to see my boat being built is a pumpout adapter to test and be sure it threads into all the deck fittings for all variations of fluids.