5.4's head design has very limited thread engagement machined into the head, and it is a long way to get to the plug. This has resulted in a bunch of spark plug blow outs, where the plug's are blown out of the head. There is a subject on this on this forum if you search, also google 5.4 engine spark plug blowouts. Folks are trying to get Ford to do a recall, but because there are so many 5.4 engines out their, the reported number of this type problem is still relatively low. Most shops will want you to replace the head at $2000-$3000, but their is a repair kit out their that installs a helicoil in the head. | 
