I went thru this on my 440/505 w/ alum housing and pump. Did the sanding on a thick piece of glass, new gasket , still leaked. I then turned the flat surface of the chromed alum. stat housing on my little lathe. It held for about 3-4 heat cycles. So then I put it back in the lathe, trued it up and cut the surface down about .006 "cant remember exactly", anyway I stopped at the inside edge of the bolt holes, creating a raised place for the gasket to ride on, around the the stat. "This will cure ur problem." my stock cast iron even leaked too..
