Changed thermostat but now constant leak around thermostat housing

Question

I went to flush my cooling system and change the thermostat on my Mk1 Golf.

All went fine until I put the new coolant in. There is a pretty steady leak from the thermostat housing.

I've tightened the screws as much as I feel safe doing, but it wont stop.

What surprised me when I was removing the old thermostat was that there was no gasket or seal around where thermostat housing attaches - only an O-ring on the thermostat its self.

Would anyone know that I can do or if I've done something wrong?

Answer 1

Found out this actually requires another O-ring, not a gasket. Another O-ring came with new the thermostat and I was able to use that.