WE just got through doing a head swap on a truck and went with one of the new style Stant thermostats that is supposed to be acceptable for use, but if you are going to use one of the things, they come with a rubber seal around the lip. Remove the rubber seal from around the lip, it comes right off. that way the metal will fit properly down into the groove in the block and the gasket and waterneck will sit down on it properly and seal. If you leave that rubber thing on there it leaves it hanging too far up and the waterneck won't seal properly and you will have a nasty leak. The groove in the block is only cut down deep enough to allow for the metal by itself. I have no idea what that rubber ring is for, but maybe they are using this thing for another application.
-------Robert
-------Robert