"O" ring thermostat housing ???

I bought a aftermarket T stat housing, new standard gasket, used sealer, flat filed and cleaned up the manifold surface.... and the darn thing leaked.

I bought a repo like tony's sells but from Classic Ind. same procedure as above...and the darn thing leaked.

I bought a gasket from Real Gaskets, cleaned off the old sealer FOLLOWED the instructions. Put the silicone gasket on dry and torqued to 20 - 25 INCH lbs haven't had a leak again!
Thermostat Housing Gasket - Silicone Gaskets - Real Gaskets Tennessee
Can not recommend them enough!

forgot to add they are reusable.