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Old 02-22-2018, 03:22 AM   #1
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Black Water Tank Outlet/Bulkhead ?

basically my black water tank has no fitting on it. just a cut hole. and it looks like whoever cut that hole messed up because it is only 2 3/4 in diameter not 3 inches (note the starter hole in the picture is 3 inches) but they cut right to the bottom of the tank. so much that the plastic is thin and weak there and it leaks, but my main question is what type of bulkhead fitting can i put on here? im not 100% sure how the original setup was. it almost looks to me the waste valve was pressed up directly to the tank? im nervous i cant expand the hole to 3 inches due too its placement. the tank appears to be an original thetford tank


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Old 02-22-2018, 05:09 PM   #2
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Many RV makers do not buy tanks with dump necks molded-in to the tank. They just cut a hole and glue-on the neck where needed for different models.
The bottom of the hole is very close to the floor of the tank to allow more complete drainage (no dam created by the bottom lip).

For your situation, I would add a neck of plumbing pipe and rebuild the system with new valves as desired.

Here is an overview of a repair that could be used as a model to bebuild:
RV Tank Repair Made Easy

Best Luck
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Old 02-22-2018, 11:16 PM   #3
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yes ok but im not going to hardline a pipe to the tank without an outlet fitting of some kind, and every type of outlet fitting i see is 3" ?
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Old 02-23-2018, 09:05 AM   #4
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Add a neck that fits the 2.75 hole (apply filler as needed)

Then use a dump valve that slides over the neck like most plumbing joints.

Best luck
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Old 02-24-2018, 12:29 AM   #5
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what do you mean by neck? just a piece of pipe? there is no need of some type of gasket or bulkhead fitting needed to be attached to the tank?
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Old 02-24-2018, 12:06 PM   #6
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The neck would be a piece of plumbing pipe glued, bonded or plastic welded onto the tank opening.

I see no way to use a bulkhead fitting unless you drop the tank and access it from the top...plus the bottom edge is on the corner of the wall/bottom, so a bulkhead flange would have to be cut...which would probably leak.

From your photo, it looks like the RV maker used a neck with flange that was tap screwed at the top and caulked all around. You could try to copy that.

Or as suggested in my first post, get a 2.75' OD pipe and bond it in like done here:
Click image for larger version

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Then glue the dump valve on the neck like a typical plumbing joint.

Best luck
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Old 02-26-2018, 06:11 PM   #7
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Crosby,

To me (an ex-Thetford engineer), that looks like the tank had a fitting welded on and it has been cut off.

If it is a Thetford tank it is probably polypropylene and there are no "glues" that will stick to it without special preparation. Your best bet is to find a local plastic fabricator that can weld it and get it done. Yes, you will have to take the tank down.

If you can't find a fabricating shop....
Next try would be West System G-flex. Get the book from them and read all about how to do it - twice.

Do not buy the plastic welder from Hazard Fright. That is an air welder and will not work on this material.

Frank
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Old 02-26-2018, 07:38 PM   #8
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Quote:
Originally Posted by F76Marion View Post
Crosby,

To me (an ex-Thetford engineer), that looks like the tank had a fitting welded on and it has been cut off.

If it is a Thetford tank it is probably polypropylene and there are no "glues" that will stick to it without special preparation. Your best bet is to find a local plastic fabricator that can weld it and get it done. Yes, you will have to take the tank down.

If you can't find a fabricating shop....
Next try would be West System G-flex. Get the book from them and read all about how to do it - twice.

Do not buy the plastic welder from Hazard Fright. That is an air welder and will not work on this material.

Frank
If the tank is polypropylene (quite possibly true) how is there no "glue" if there are materials readily available like this 3M/Loctite system?
https://www.homedepot.com/p/Loctite-...1925/100371829
Quote:
Bonds well with polyethylene and polypropylene surfaces
Please take a second look at the photo. The outline of the square flanged neck and adhesive all around the edge with a tap screw hole at the top is clearly visible with no tooling marks like from a cut.

Of course having a professional fabricator add a neck is surely a great idea...we agree about that.

Best luck to the OP
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Old 02-27-2018, 01:40 AM   #9
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thanks for the input guys.

well ill also mention both of the original waste gates on both the grey and black tanks failed at the same time, so while i removed all the plumbing thats where i got confused,

my grey water tank has a male threaded 'outlet' already attached (sealed very well) then there was 3 inches of threaded pipe then the thetford waste gate attachment.

but on the black water side the waste gate was just pressed against the black water tank (giving that outline) so with your input maybe it was 'welded' and that weld failed, because i do remember it being a bitch removing that side of the plumbing.

which is why i thought i was dealing with a system that was already altered by a previous owner, or as mentioned earlier i am still confused why the hole on my black water tank is so small when the gate and all fittings were for a 3 inch system.

thanks again for the input and discussion
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Old 02-27-2018, 07:52 AM   #10
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Scarab,

Please read what I wrote carefully. Then read the instructions (available on line) for the referenced Loctite product. You will see the my advice still fits.
A special surface treatment is both suggested and required as it is the "Activator" part of the kit. I also don't believe that Crosby will be able to reattach that flange with 2 grams of adhesive and only one chance to get it right. This was why my suggestions were as they are.

If I were Crosby and I could not find a fabricator to do the repairs properly, I would get an N2 bottle, a regulator and a plastic welder an go at it. But then again, I did do that a Thetford.

Frank
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Old 02-27-2018, 10:06 PM   #11
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there is a thetford manufacturing sticker on the bottom of the tank but the rest of the info has been worn off, rv is a 73
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Old 03-23-2018, 11:58 AM   #12
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If it were mine I think I would have a fabricator take a 6"x 12" x 1/8" stainless steel bend it into a L, have a stainless 3" threaded bung welded on then bolt the L to the tank side and bottom with stainless steel nuts and bolts. I would use a generous amount of 3m 5200 to seal it to the tank to eliminate any leaks.

This fix would be much stronger then original and shouldnt cost much.
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