Thermostat housing

[richardsMk8]

Help. I drained my cooling system. Replace the thermostat, and an oring, for the thermostat housing, added coolant. Now, it's leaking between the thermostat housing. Why is this leaking, between the housing? Am I supposed to add a gasket, too?

 

[xtriggerman]

I dont remember if there was a gasket or O ring in there but my leak was from the rusted/corroded pipes on the housing. Heres my fix from 5 years ago
Repairing a rotten lower Thermostat Housing You really need to pull the hoses off entirely and give it a complete looking over for rot.

 

[TOM MKVIII]

There's an o-ring between upper and lower thermostat housings.