[ShrimpBurrito]

I just measured the thickness of the cylindrical part of my spare seal (the part that sits above the flange), Thetford part #33364, and it is 5/16" thick. That is the one I have always used, and I have never had a problem under ordinary circumstances (the exception was a very rough road that knocked the toilet off the flange a bit). My guess is that is the one that came with the valve, so that is the one I would use.

Before you put the valve on the toilet, I recommend lubing the slide (through the discharge hole) with some Dow Corning 111 Valve Lubricant and Sealant. It makes the slide super easy to operate, and also helps the seal last longer -- I think the seal gets deformed when the gate of the valve grabs the seal, due to inadequate lube, and bunches it up when you try to close it, which eventually causes it to tear. The Dow lube has roughly the consistency of Neosporin....maybe even petroleum jelly. It comes in packets, toothpaste-sized tubes, and caulking tubes IIRC. I got the toothpaste-sized tube for ~$15 (I think on eBay), and it will undoubtedly last me as long as I have the trailer.

Note in the future, you can buy a gasket kit (Thetford part #09872) for the EZ Slide valve for about a third of the price of a new valve, so don't throw your old valve away if it is not cracked. Get a seal kit for it and you can make it like new, and when you're putting those seals in, you can probably do a better job of lubing it with it disassembled.

Dave