Welding a Black Tank Crack

The threaded plastic fitting in the black tank for the toilet ring has broken away from the tank a full 180 degrees around the aft side. I think this happened because the tank itself slopes down and away from the floor as you go aft, which put a lot of strain on the weld (the tank ring, as you screw it in, pulls the aft side of the tank up). It looks like the fitting ought to have a wedge of plastic under it on the aft side to reduce this stress.

The break is clean, no cracks into the skin of the tank as far as I can tell. It looks like the fitting just tore out of the tank along the seam, with only minor pieces of the tank adhering to the fitting.

The photo is looking down on the fitting–this is a mid-bath and the toilet faces forward in the trailer.

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The thing that kills me is that I checked for leaks and this problem didn’t show up until the Sovereign was under way with a full black tank–the slosh resulted in a spill that leaked over the top of the tank, soaked the fiberglass insulation on the top of the tank, then dripped out the belly pan with no hint of a leak around the toilet inside the trailer. Fortunately I had filled the tank with clean water just in case there was a leak at the top of the tank.

I’m going to attempt to fix this in place. The options are using a Scotch-Weld product (it’s a glue, has about 2 minutes working time and is purported to generate a bond stronger than the base material) designed for plastic, or do a plastic weld, which I haven’t done before. Has anyone used the Scotch-Weld?

As far as doing a plastic weld, does anyone know what kind of polyethylene the tanks are made of? Like (HDPE) High Density Polyethylene, (HMW) High Molecular Weight Polyethylene, (UHMW) Ultra High Molecular Weight Polyethylene, (LDPE) Low Density Polyethylene, (LLDPE) Linear Low Density Polyethylene, (ULLDPE) Ultra low Density Polyethylene, (MDPE) Medium Density Polyethylene? I guess you’re supposed to know this when you order the plastic welding rod.


Up front, let me just say I haven’t done a water test, but the repair seems very sturdy. That being said…

I cut a final set of gussets and shims out of LDPE. You can see the outer diameter of the fitting inside the tank is much smaller that the diameter outside. The shims go under the flange to provide the proper angle between the tank skin and the fitting (or what I perceive as the proper angle). There’s no way to tell unless I drop the tank. The gussets are 1/4″ thick, so there is plenty of height, even with the shims in place, to get a good glue joint to the edge of the flange.

I did note yesterday that in taking the fresh tank out of the Safari, the top was not flat, but drooped. That may be the case here, which means the top skin would come back up “flat” once the tank is full. But that’s water over the dam, as they day.

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The white in the lower left portion of the clamp photo is a stiff piece of paper on the face of the ceramic heater. I applied heat locally for about 3 hours. When I look at the photos, it makes me think “that inner gusset can be inserted from the top?” Yep. Just use several pairs of surgical gloves to keep the very Nasty smelling Scotch Weld off the skin. I think I had 25% of the glue on the gloves…

BTW, if you decide on using Scotch Weld, get plenty of the 10:1 mixing nozzles. I bought six and with all the testing and self-doubt, I used 4 to get to this point. I wish I had bought 10. (If you’re careful, you can re-cap the glue after using only a small amount from the dispenser, so the mixing nozzles seem to be the limiting factor in getting the most out of a $30 tube of glue.)


Finally got warm enough to hook up and go test the tank. If you recall, the problem in detecting the crack in the black tank is that I didn’t overfill it, so any problem on the top skin would go undetected until you started sloshing down the highway.

So my test rig had to enable filling the tank above the top. Here’s the solution–it appears than almost any hardware store will have the male PVC fitting that has the same threads as the toilet flange. With the toilet flange removed, just screw in the “test device” and fill to about 6″ above the top of the tank. 6″ inches of water is not a lot of pressure (about 0.25 psi)*, but that puts about 200 lbs of force on the top skin from the inside, which is enough to flex it a little and open any cracks.

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After five minutes, no detectable leaks! The Sovereign is practically ready to roll to Burning Man! Now we’ll see if the glue can handle the stresses of the slosh over many miles…