Sealing around a tank boss

Aug 19, 2025 Last reply: 11 months ago 7 Replies

I’m trying to fit a tank boss to a motorhome waste water tank.



Eg.

formatting link
It’s a regular flat sided tank so I was hoping it would be straightforward but, as is often the way, it hasn’t turned out that way.



The boss comes with a rather soft rubber washer that I assumed went under the nut on the outside, but as soon as I started tightening the nut, it just “oozed” out from behind the nut. Perhaps it should go on the inside?



Anyhow, I tried just tightening it all up without the washer but there’s a slow leak around the fitting.



Should I



a) try it with the washer on the inside?



b) try it with the washer on the outside and just resist the temptation to over-tighten?



c) try it with a different/harder rubber washer?



d) slather it all in some sort of sealant before tightening it up?



or something else?



Tim


Doh. Answering my own question. Shouldn’t have made assumptions about fitting it without checking first.

A quick watch of some YouTube videos suggest that a) is the right answer….

Tim

Tim+ snipped-for-privacy@yahoo.co.youkay> wrote in news: snipped-for-privacy@news.individ ual.net:

Yes, inside is correct. Even if you could apply approptiate pressure, water would still pass to the outside and leak along the thread of the nut.

A general rule is you want the available pressure to help you. Here it's the pressure of the water inside the tank - by putting it on the inside the pressure is acting to seal the washer against the side, while on the outside the pressure is trying to push the washer away from the side. You are also applying pressure with the nut to counteract it, but it's better to have the sum of the pressures than their difference.

Theo

I've always thought that it should go on the side that doesn't turn thus inside & outside may be the wrong terminology.

Remounted it with the washer inside the tank and all seems well this morning.

Tim

That is the expected result.

-- Labour - a bunch of rich people convincing poor people to vote for rich people by telling poor people that "other" rich people are the reason they are poor.

Peter Thompson

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required