Forespar Marelon Threads . . . Really Confused

[Hello Below]

I am replacing a totally inadequate and I believe unsafe raw water strainer that came fitted in my boat when new. The engine is a Yanmar 3YM30. After some legwork, I decided on a Forespar MF 810 strainer. That strainer has 2 - 1 ½” ports for the inlet and outlet. I acquired 2 - Marelon (910063) male to female reducers to go down to ¾”. I also purchased a Marelon (905007) ¾” male to barb tailpiece to connect the outlet hose and a Marelon (901001) ¾” male to barb elbow for the inlet hose.

I put 2 – 3 wraps of Teflon tape on all the fittings. I installed the 2 reducers, first hand tightened and then 1 – 2 turns with a crescent wrench to what I felt was a good torque. I similarly installed the elbow except had to use lock joints rather than a wrench to tighten. All of these 3 seemed to have made a good solid tight mechanical joint. Then it came to the elbow. I threaded it into the reducer and found it very sloppy. I easily hand turned the fitting all the way into the reducer, with little resistance. It only stopped when the 90 deg barb butted up against the reducer and physically couldn’t turn any further. I had to back the elbow out a few turns so that I could fit 2 - ABA SS clamps on the hose. In that position, the elbow could easily be swung back and forth with one finger.

Being an optimist, I cracked open the thru hull and within seconds of the water reaching the strainer, I had sea water leaking out the inlet (elbow) fitting. I removed all the fittings, cleaned off the Teflon tape and replaced it with Permatex pipe thread sealant with Teflon. After allowing 72 hours for the Permatex to cure, the results were identical: 3 solid fittings and 1 very sloppy and leaking elbow.

So I researched the Internet some more and found a Forespar document that states: “Marelon plumbing fittings have NPT parallel threads. The threads can be sealed with Teflon tape or plumber’s dope.”

Well I have news for them regarding the latter. So I have a number of questions:

1. What the heck are NPT parallel threads? I thought the “T” in NPT stood for tapered and parallel threads were NPS?
2. I though you used Teflon tape on tapered threads vice parallel threads?
3. If Marelon plumbing fittings are all the same, why was I able to torque 3 of the 4 fittings but not the elbow?
4. How can I make the 4th fitting physically solid and leak proof? Could I or should I use 3m 5200 or West System 2 part epoxy?

A plumber I am not. Any advice would be appreciated.

Thanks.

Len

 

[kito]

I went through this hell myself. I wish fitting vendors would be more clear on the thread type. Just guessing but I believe your reducers and tailpiece are NPS threads (straight). I have never seen a male elbow with a NPS thread other than a flare fitting. I think you are trying to screw a npt male elbow into a female nps reducer.
Btw, the NPS fittings I used, like a female tailpiece hose nipple, had a gasket for the male fitting to seal against since the threads are not tapered and don't seal well.

 

[Bill Roosa]

National pipe thread (NPT) are the kind that you find on iron pipe used for gas and plumbing lines. They are tapered so that you can seal them. Male parts and female parts both have the taper. NPT parallel threads are weird and can only be used with MASSIVE amounts of tape and sealant. As you found out that has some problems during the use of said threads.
I would recommend metal parts for this application.

 

[Benny17441]

I was just going to say the same thing, choose either brass or bronze fittings as they will not interact with the Marelon to induce any galvanic corrosion.

 

[Hello Below]

I went through this hell myself. I wish fitting vendors would be more clear on the thread type. Just guessing but I believe your reducers and tailpiece are NPS threads (straight). I have never seen a male elbow with a NPS thread other than a flare fitting. I think you are trying to screw a npt male elbow into a female nps reducer.
Btw, the NPS fittings I used, like a female tailpiece hose nipple, had a gasket for the male fitting to seal against since the threads are not tapered and don't seal well.

I would have thought that the tailpiece and the elbow would have the same thread. Yet the tailpiece snugs up nicely in either of the 2 reducers, with or without tape. The elbow was loose in both of the reducers. A mechanic friend did look at the elbow and tried to show me why it was a parallel thread. I never did see what he described. And I also thought that nps threads used o rings but neither the reducer nor strainer is designed to take a gasket as such.

 

[Hello Below]

National pipe thread (NPT) are the kind that you find on iron pipe used for gas and plumbing lines. They are tapered so that you can seal them. Male parts and female parts both have the taper. NPT parallel threads are weird and can only be used with MASSIVE amounts of tape and sealant. As you found out that has some problems during the use of said threads. I would recommend metal parts for this application.

Bill please define MASSIVE. At least wrt the Teflon tape. Two to 3 wraps didn't do the trick, at least on the elbow. And I did use ALOT of Permatex on all 4 of the fittings. But that only resulted in the excessive goop all over my arms, hands, shirt, pants, strainer, etc. I may end up using bronze or brass fittings. But it frustrates me that Forespar shows all these fittings and the strainer on the same page in their catalog. Maybe 6 or 7 wraps will suffice. The Marelon strainer and fittings (ABYC approved for below the waterline) are mounted at or near the waterline so I want to get this right.

 

[kito]

Just me but I would never rely on multiple wraps of Teflon on a below waterline connection to keep my boat afloat.

 

[woodster]

Bill please define MASSIVE. At least wrt the Teflon tape. Two to 3 wraps didn't do the trick, at least on the elbow. And I did use ALOT of Permatex on all 4 of the fittings. But that only resulted in the excessive goop all over my arms, hands, shirt, pants, strainer, etc. I may end up using bronze or brass fittings. But it frustrates me that Forespar shows all these fittings and the strainer on the same page in their catalog. Maybe 6 or 7 wraps will suffice. The Marelon strainer and fittings (ABYC approved for below the waterline) are mounted at or near the waterline so I want to get this right.

use bronze fittings .......not brass....... on the sea water intake brass will deteriorate in about a year......... again bronze only