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Old 20-08-2018, 16:03   #1
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Join Date: May 2014
Location: Houston, TX
Boat: Catana 471
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Leaking flange on top of plastic holding tank

New-to-me boat has two plastic holding tanks, and both have a VDO level gauge/sender that screws into a mounting flange on top of the tank (see photos). On both tanks, fluid leaks out from between the mounting flange and the tank surface.

The mounting flange is apparently attached to the top of the tank with self-tapping screws and a rubber gasket. I haven't taken it apart yet, but I suspect that self-tapping screws were overtightened and threads failed, so the gasket simply isn't getting compressed enough to form a good seal. Apparently this has been an ongoing issue, since the PO attempted to seal around the perimeter of one of them using caulk or 4200 or something similar... which didn't work of course.

Question is... is there a better way to mount these flanges to the top of the tank so that the mounting screws can be tightened down and the gasket compressed better? Something like this adapter from WEMA seems ideal, but I haven't found one in the correct size. I could presumably get some nuts and washers on the underside of the tank surface, but I'm afraid the washers would protrude into the thru-hole and interfere with the sensor being screwed into the flange.

Should I use a sealant in addition to the gasket?

Or possibly a different approach. The sender has 2-1/2" stamped on top of it (seen it labeled elsewhere online as 64mm). I don't know if this is BSP or NPT, but if NPT, could I drill out the hole to 3", insert a 3" Uniseal plus a 3 x 2.5 Spig x FIPT bushing, and screw the sender into that?

Much thanks!


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Old 20-08-2018, 17:19   #2
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Re: Leaking flange on top of plastic holding tank

Quote:
Originally Posted by Jetx View Post
Or possibly a different approach. The sender has 2-1/2" stamped on top of it (seen it labeled elsewhere online as 64mm). I don't know if this is BSP or NPT, but if NPT, could I drill out the hole to 3", insert a 3" Uniseal plus a 3 x 2.5 Spig x FIPT bushing, and screw the sender into that?
Ooops, I blew that one. Found another drawing online, and turns out the inner hole in the mounting flange, which the sender screws into, is a 1.25" BSP thread. (No idea what the 2-1/2" marking refers to.)

But the fundamental question remains: is there a better way to mount this flange than simply using self-tapping screws, so it seals properly to the top of the tank?
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Old 20-08-2018, 17:48   #3
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Re: Leaking flange on top of plastic holding tank

The simple solution: replace the gasket. NO sealant! They don't leak as long as the screws aren't overtightened.



And if you never want to have to remove the cap again to remove the senders to clean 'em, replace your tank level indicator(s) with one of the Profile systems Profile Tank Monitors , also sold under the private label brand name SCAD, top rated by PS a few years ago. The senders go on the outside of tanks, so they never come in contact with the contents...which means they can never get clogged with the animal fats in waste.


-Peggie
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Old 21-08-2018, 11:00   #4
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Re: Leaking flange on top of plastic holding tank

Got it. And simple enough.. thanks Peggie!
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