The O ring should be on top of the T-stat.
Some non Ford T-stats are too small in diameter so the O ring falls
into the space around the T-stat instead of sitting on top of it.
If that's the case there is no "crush" on the O ring and ..................leak.
97 LX sport 4.6
T-45 5 speed, 3.73 with track lock; PI heads, 02 MAF, #24 injectors,190 fuel pump,JBAs, 75mm TB. ect,ect.; Aluminium drive shaft. 13" PBR Brakes; Dennis chip, lowered with solid sway bar bushings and hiem joints; console mounted E brake WITH the cup holders; C&l Plenum, Steeda U/D; VERY big radiator; 2.5" duals, Magnaflows;B&M ripper shifter; bla..bla...bla...
Lincoln MKVIII 93.........sweet ride.