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dbj216 · 05-01-2020, 07:11 PM

I worked in the polyethylene tank molding factory for 4 years. I can spin weld, but you wouldn't want me to. It is a process that takes some time to master, like how much pressure and when to quit spinning.

Since I'm a lousy spin welder I use rubber grommets in my tanks. We used these things at the factory to seal fuel and hydraulic connections in the tanks we were making. Inca Plastics has been rotomolding tanks since the 1960s for the RV industry. They sell these grommets with confidence. I might add I do not use them for the 3" drain port. I use the molded boss that usually comes with the tank. I have installed grommets on my 1966 Trade Wind new tanks, on my 75 Overlander new tanks and now on my friend's 76 Sovereign existing tanks. All of these were top side drain water inlet locations. I also use a 3" grommet for the toilet connection. They are easy to install, are a press fit to the tank and a press fit to the ABS piping. Very tight and perfectly adequate for "splash" seal. I would not hesitate to use them on the bottom of the tank if there was a need.

David

Bauxter · 05-02-2020, 07:55 AM

David, Thanks. Looking at your tank pictures, you are displaying a couple of my problems. The toilet flange looks to extend a few inches above the tank. My tank is snug to the bottom of the plywood floor, so I have 3/4" to connect the tank and toilet. The 3/4" thickness worked on my '73 trailer with ABS tanks, but I can't see that working with my poly tanks and grommets, as I also only have 3/4" to work with on my 74. I think I could do the grommets for the 1 1/2" vent connections though.

Additionally, my tanks are 8" deep, so I need to keep them as close to the plywood deck as possible.

Thanks for the feed back.

FYI. I did eventually find a RV customizer that has the tools, in Central Ohio. Anyone needing that contact info, PM me.

Mike

dbj216 · 05-02-2020, 07:34 PM

Here is a photo of a broken spinweld I found when I disassembled my trailer. Maybe it broke when someone tried to remove the threaded floor flange. Someone tried to fiberglass it back to the tank. It was leaking when I bought the trailer. A photo shows how I found it.

My new black tank is also mounted flush with the plywood subfloor. The second photo shows the rubber grommet sticking up about an inch and a half above the tank.

The floor flange I used was about 3/8" thick if memory serves. So I made a spacer to get me flush with the top of the grommet and floor flange. The toilet was a tight fit into the grommet by about 3/4".

I'm not a fan of molded plastic pipe threads. And plastic toilet flanges with 3" male pipe threads aren't as common as they once were. Seems like everyone is sealing on the ID of a 3" ABS pipe.

Glad you found a spin weld source.

David

Bauxter · 05-03-2020, 05:43 AM

I am not sure I can ask this correctly, but here goes.
If the grommet protrudes above the tank 3/4" or less, and the toilet flange can connect directly into the grommet, within the height limitations of the plywood (3/4"), allowing the toilet to sit directly on the vinyl covered 3/4" deck, I would like to try it, as long as I could use the same hole for a spin weld, in case the grommet will not work. A lot of IF's....
Is there a grommet brand that I could buy that is low profile, that you recommend. Maybe spending a few bucks to see it in person would be the best way to evaluate the situation.

dbj216 · 05-03-2020, 07:18 PM

Good idea. Purchase a 3" grommet from VTS or someone and then you can "layout the dimensions" and see if it would work for you. Some RV toilets are now made without a "down pipe". The bowl just dumps to the floor flange. Kinda like having a black tank in the toilet that eventually drips in to the real black tank. Thetford Aqua Magic I believe it built this way. I don't like that design. I insist on the down pipe molded into the toilet so it will engage the grommet.

It does require some study to see if the grommet solution will work for you.

David