Thermostat neck from Billet Technologies

71DodgeDemon340 said

Id try to put some gasket sealant around the o ring first.

Permatex #2 or Versa Chem type 2 should work

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And then when you take it apart at some future date, the o-ring will be ruined (you'll never get the RTV off of it). The o-ring is the seal. It needs no sealant.