Just want to leave a tip for anyone else who runs into a leak issue after installing a new thermostat on their 3F-E 80 Series. It's not clear from the FSM, but you drop the thermostat into the lower housing and then the lower/upper housing gasket over the thermostat flanges. I had a noob moment my first go-round, when I set the gasket and then the thermostat on top of the gasket, which led to a leak. Also, don't forget the smaller round gasket on top of the thermostat. 
