Rxfire--

I had my n/a C223 leak from the thermostat housing and tightening it did no good. So, I removed the t'stat housing from the head and found some corrosion on the gasket faces. But, worse, I found that the t'stat housing gasket face was badly warped; ie, not flat. It is a fairly thin section and there are only four bolts to hold it down, looks like a recipe for warpage.

What I did was to use a large piece of very sharp abrasive paper on a flat plate to cut the surface down to flat, and this cured the leak. This had to be seven plus years ago.

I like Permatex #2 for gaskets like this.

Paul