All too often, the thermostat housing will be rust-pitted badly enough that its very difficult to seal, no matter what gasket/sealant combination you use.

Try to find an un-rusted thermostat housing.....its possible to re-machine a pitted one, but its a surprisingly difficult setup, either in a lathe or Bridgeport class mill.

cheers

Carla

it sound as if it is slightly warped, when it is hot the gap closes up , when it cools down it opens up. check it with a straight edge , if it is warped carefully put it in a vice and run a file over it

Only three things really that can cause a thermostat housing to leak.  The gasket, the hose clamp not tight enough, or it can be leaking up the threads on the bolts and coming out at the top.  I have had leaky bolts and just used some thread sealant on them. Are you sure the thermostat is the right size and fitting properly.

The only other possibility that occurs to me at the moment, is if you have a porous spot in your thermostat housing.

Regards,

Pat O'Connor

I forgot to mention this is on a 39 flat head. I will try using no gasket. It just baffles me why it leaks when cold but not when it is hot. Its not the hose. It leaks right where the housing meets the head.

I've had good luck with blue Permatex and NO gasket to join my themostat housing to the head.

You didn't mention what type of sealer you are using; maybe you've already tried this?

If not, follow the directions on the package, and just tighten the bolts until the Permatex starts to ooze, then stop.

Regards,

Pat O'Connor

Are you sure it isn't the hose?

I recently had my engine rebuilt and I put a new thermostat in, just before I stated it. It didn?t leak when hot but when cold. I replaced the gasket and put sealer on it. It leaked again, but only at night after the engine cooled off. I then replaced the thermostat again with a new gasket and more sealer. It still leaks when the engine cools off! Any ideas what I?m doing wrong?
Thanks for the help.