stinking water neck driving me crazy

I have haven't had any trouble with leaking thermostat housings since the 70's. My Dad showed me how to flat file them, to check the surface on the intake and to not over tighten the bolts. I always use a gasket with a thin layer of copper silicon on both sides then just snug the bolts good. IIRC the bolts are 3/8" and a lot of people really get down on them and distort the housing.