97 F-150 4.6 motor. Does the o-ring go into the intake housing first, or does the thermostat go in first, an the o-ring goes on top of the thermostat? A buddy and I are having an disagreement here, an I'll be damn if I'm pulling the elbow off to find out! I'm telling him the o-ring goes in first, then the thermostat. Then when the housing goes on, when tightened, the housing will push the thermostat down onto the o-ring an seal properly.

 

Oh well, I found out! I was WRONG! O-ring goes on top. Now I gotta go tell him he was right. Man, I hate this. Do I really gotta do this!

 

Yes in both the 5.4 and the 4.6 the "O" ring goes above the thermostat. However it looks to be a mistake because of the fit. When you open your thermostat pack most of the rubber gaskets are around the periphery of the rim of the unit.
If you try to mount it like this it won't fit or fit properly.

I have also added a bead of "black" RTV sealant between the rubber grommet and the thermostat for good measure.