Thetford valve adapter so you can use a Valterra

I found that our leaky Thetford black waste valve was glued into the plumbing, so had to cut it apart to take it out. Thetford valves are expensive and hard to find. I made this adapter to go onto the white plastic flange on the OEM black waste tank and adapt it to regular 3 inch abs plumbing. It's made out of 3/8 inch thick black ABS, 5.25 inches square, bought from TAP plastics. ABS is extremely easy to work and really cheap. I glued a short stub of 3 inch ABS into the hole. In hindsight, I should have made the stub a little longer, but it really depends on how your installation was done and what you're replacing.


If you have to cut it apart, leave as much length as possible on the parts you plan to keep, even if you have to cut through the middle of glued up couplings. You can always grind off the coupling to get down to the remaining pipe and get to a stub you can use. ABS glue is great for filling voids.


I placed the hole for the 3 inch stub a little off center so it's at the bottom of the opening into the black tank and clears the hole in the frame rail.


A rubber 4 inch sink drain gasket fits the OEM flange perfectly. I also used some RTV on both sides of the gasket, just in case. So far, so good.


In the OEM installation, the top two fasteners are wood screws, the bottom two are 1/4" bolts. I replaced all that rusty mess with stainless. I also used stainless washers, not shown in the picture.


If I hadn't already had a forstner bit to make the flat recesses in the corners where the screws/bolts are, I'd have used a thin piece of ABS glued to the face of the 3/8 inch piece, cut to the appropriate shape.


The cost of parts to make this was much less than the difference in price between thetford and valtera valves. I used a mission coupling (rubber coupling with a stainless sleeve) between this and the rest of the plumbing, so it's easy to remove in future.