Thermostat replaced less than 5 months ago -- now leaking

[Fless]

IIRC there is a small variety of water pump and thermostat housing pairings, and that may account for some slight mating differences. I used a new AC Delco Pro water pump but kept the OEM (I believe) thermostat housing, so not necessarily a perfect fit combo.

 

[Larryjb]

I was unable to seal a thermostat leak until I used some RTV. There is a special type of RTV for thermostats that I used. I used some on the new water pump as well. If the thermostat were on the top of the engine so that I wouldn't lose half my coolant everytime I had to remove it, I might have tried several other methods.

 

[JetBlackNali]

Pic is way small. What is it?

Permatex Aviation Form-A-Gasket® No. 3