i removed the old, not working anymore, thermostat and have the new one which came with two o-rings but the old thermostat had no o-ring on it. the thermosat housing is plastic, do i need an o-ring on the new thermo if so where would it go in the installation process?

Atually there should only be one ring in there and it goes like this:

1:-pump housing
2:-thermostat
3:-o-ring
4:-thermostat housing
5:-screws
6:-lunch