Electric Fan Thermostat Leak

[Darthbaker]

Looking for a bit of advice on my 71.

We installed an new Fan Thermostat in the motor block this past weekend. There were no leaks from the it for a few days. Yesterday afternoon I noticed it was dripping a little anti-freeze (not a large amount). We've tightened the inner and outer housings about a tight as they'll go - the outer housing (that screws into the block) could be tightened a little more but being brass we don't want it super tight ...fears of stripping it. The leak remains.... Any advice on how to stop this minor leak? I've been told NOT to use plumbers tape or anything like that on the threads. (Even said it on the installation directions)..

Curious as to what everyone else has done to prevent/stop this from occurring.

Thank you in advance for any advice.