I just swapped a 180 degree into my 4.6L, and it's been quite a while since I changed a t-stat, I was quite surprised to see how they installed the o-ring. I'm used to the "old days" when you had gaskets. I removed the housing and looked for the o-ring on the upper housing and seen none..... then upon looking at the stat, I saw it just laid on top of the stat. Well, I was hopeful that it was something that usually seals OK; but by design, I figured is has to be a pretty reliable method. Probably one of the simplest jobs you can do on a new vehicle.
2005 F-150 Supercab 4.6L Auto w/3.55 LS
Mods so far: Magnaflow SIDO catback, AEM Brute Force CAI & Gryphon w/ custom tunes by Bill, Tinted Windows, H-D Black Headlamps, FX2 lower valance.