This Thetford valve flange has a spigot on the upstream tank side. It is designed to be inserted into a socket in your black tank that is approximately 3.5" inside diameter. Any of the Thetford flat top valves like the 08706 can then be easily bolted to it.
In most cases, the tank it fits is made of polyethylene and so a solvent weld cannot be made. It is most common to hold the spigot in the socket with a hose clamp like our VTS-449.
Made of ABS plastic.