Holding tank- seal leaky threads

My plastic holding tank has the pump out fitting on the lower side of the tank. The female threads must be worn and won't seal well even with a lot of thread sealing tape.

Will 4200/5200/ seal this up?

I have not tried 5200 on plastic, don't know if it will stick. Rule Elastomeric caulk does stick to plastic, not sure how it will hold up to the contents of the holding tank. I have used it on a Triumph boat which was made of LDPE and it worked well. Rule says that it will stick to King Starboard.

We had exactly the same problem with our new tank 18 years ago. It did not matter how much or little sealing teflon tape we put on I could not get it to stop weeping. It was a good thing it was a new tank and I tested it with water, out of frustration I finally grabbed the "good" old 3M5200 and viola sealed. Like I said 18 years and never had a drip as I have some cloth under the fitting to be able to see a leak.

I also at the same time put a plug in the lower tank port and use a tee in the top deck pump-out for the macerator overboard pump. The reason was if the pump were ever to fail it would be with a full tank according to Murphy, now with the pump on the top fitting it doesn't matter. No I am not scared to use 5200, it has been a great friend over the years just like butyl tape and other sealers are.

The cause of those leaks in plastic are typically due to over tightening. The good news is as its waste either 4200-5200 or even sikaflex 295 etc will seal adequately.

Edit: In case it's not obvious take apart and clean the fittings and use the sealant as you would a standard pipe dope, slathering a bead around the fitting I'm sure "may" work but Murphy won't let me recommend it.

3M Sealants

If you look at the 3M Sealant Chart attached it looks like 4000 is a better choice for plastics

I think I just answered the same question in a PM... Don't even THINK of using 5200 or anything remotely like it on ANY sanitation system fittings. In fact, no sealant at all should be used in sanitation system plumbing.

rochepoint said:

We had exactly the same problem with our new tank 18 years ago. It did not matter how much or little sealing teflon tape we put on I could not get it to stop weeping. It was a good thing it was a new tank and I tested it with water, out of frustration I finally grabbed the "good" old 3M5200 and viola sealed. Like I said 18 years and never had a drip as I have some cloth under the fitting to be able to see a leak.

I also at the same time put a plug in the lower tank port and use a tee in the top deck pump-out for the macerator overboard pump. The reason was if the pump were ever to fail it would be with a full tank according to Murphy, now with the pump on the top fitting it doesn't matter. No I am not scared to use 5200, it has been a great friend over the years just like butyl tape and other sealers are.

Click to expand...

The average working life of ANY hose is about 10 years...after that they start to become hard and brittle and begin to crack. How do you plan to replace yours--which, after 18 years are WAY past due and any connected to to below-waterline thru-hulls actually put your boat at risk,btw--without destroying the tank and anything else to which you've bonded them with 5200?

There are 3 ways to do anything: the right way, the wrong way...and what you've gotten away with.......................so far.

meridian said:

My plastic holding tank has the pump out fitting on the lower side of the tank. The female threads must be worn and won't seal well even with a lot of thread sealing tape.

Will 4200/5200/ seal this up?

Click to expand...

It's hard to imagine the treads being worn because in this application the fitting was probably installed and tightened only once.

There have been several suggestions, some may work, some may not. I'll give you a "last resort" idea. Buy a fitting relocation kit from your favorite marine vendor. Then devise a plug for the existing hole that would have a gasket on each side and a bolt holding the two sides against the tank and gaskets. There are other ways to plug the hole, use your imagination. Visit a plumbing supply house, they may have something that would work.

Install the plug and use the kit to make a new fitting nearby.

When tapered plastic fittings will not seal in spite of wraps of pipe tape, the problem is often that the female fitting has a crack that opens/leaks when the male fitting is screwed in and closes to an invisible hairline when it is removed. The crack is normally at the bottom where it is most difficult to see!

rochepoint said:

I also at the same time put a plug in the lower tank port and use a tee in the top deck pump-out for the macerator overboard pump. The reason was if the pump were ever to fail it would be with a full tank according to Murphy, now with the pump on the top fitting it doesn't matter.

Click to expand...

I've done something similar, twice. My tanks don't have a top opening you can pump out from, but the lower opening has just an elbow, straight up to a tee above the top of the tank. The macerator is plumbed into that tee, above the "water level" of the tank, along with the deck pump-out above. Murphy ensures that a macerator will never fail with the tank empty!

Shoalwaters said:

When tapered plastic fittings will not seal in spite of wraps of pipe tape, the problem is often that the female fitting has a crack that opens/leaks when the male fitting is screwed in and closes to an invisible hairline when it is removed.

Click to expand...

Mike beat me to it. I found this exact situation when I bought this boat. One of the (many) things that led to the complete replacement of the waste systems of both heads.

I also have to agree with Peggy. Any time I've encountered "goo" in a plumbing system, it's been the result of a half-assed, or at best an emergency, repair. And always requires parts to be ripped out and replaced.

+3 for a crack. there is no way those fittings wear out. There is little or nothing that will stick to certain types of plastic often used in tanks.

I would try bonding it place If you can live with the fact of its never coming out again

PL glue should do it.Even with a metal pipe to hose fitting.

Probably it has simply been over tightened .

Purchase a 1 1/2 in pipe tap (if that is the size) and simply re tap the fitting , creating new deeper threads .

If you can not purchase or borrow a tap, buy a right sized steel pipe nipple .

By cutting 3 or 4 groves in the threaded portion with a sharp hack saw a po-boy tap is created.

Use it slowly , remove it frequently and it will cut new deeper threads in plastic just fine.

Purchase a new hose fitting (biggest threads) and assemble it with the YELLOW teflon tape.

At the store take any female fitting off the shelf and see weather a plastic or metal fitting is larger , goes in less, to fit your slightly over sized new threads.


Yellow is made for fuel or gas and is far thicker and tougher than the thin white crap at ACE,

When dry finished I would shmeer a layer of clear silicone over the joint.

This will seal W-T , yet is easy to remove , should it be necessary when fitting a new hose.

If possible I would install the new hose on the fitting , before screwing the fitting in place .Secure other end of the hose last. Hand tight is all it takes.


Have fun!

The Headmistress suggested if I have enough of the fitting protruding from the tank I should try a hose clamp after installing the elbow. IT WORKED! I've plugged the two side 1 1/2 holes and will use Uniseal fittings to bring both in and out pipes through the top of the tank.