From this thread, which is stickied at the top of this section:

http://www.grandprixforums.net/trann...ion-10095.html

Heres the low-down on cooler line fitting thread sizes.....

97-Most 99s:

Cooler FEED fitting (points straigth down) 1/4-18 NPSF, NO CHECKBALL
Cooler RETURN fitting ( Horizontal-points towards front of car) 1/4-18 NPSF


Some 99s, all 00-early 04s:

Cooler FEED fitting (points straigth down) 3/8-18 NPSF, HAS CHECKBALL
Cooler RETURN fitting ( Horizontal-points towards front of car) 1/4-18 NPSF


Early 04 to current:

Cooler FEED fitting (points straigth down) 9/16-18 UNF, NO CHECKBALL
Cooler RETURN fitting (Horizontal-points towards front of car) 9/16-18 UNF