Thermostat Housing

Installed properly? Turn the thermostat housing upside down . Place the thermostat in the housing recess . Use contact cement and glue the gasket over the thermostat and housing . Let it set up then apply a light coat of gasket sealer to the dry intake.

Not saying you did but many times the thermostat is not in the recess built into the housing correctly. Then they always leak.

The o-ring style are great to use if the intake has a perfect flat surface. Most times on older intakes they do not.