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My '98 Expy w/5.4 needs new plugs. I bought the car used and had the plugs changed when I first bought it. My then mechanic didn't mention any potential problem doing it.
I've since moved to a different state and my new mechanic, who has done other work I've been real happy with, says it's fairly easy to strip threads on the head changing plugs and that there is even a "kit" available to deal with the problem. I'm about to take a 2000 mile trip and he recommends changing them after I return. Doesn't want to see me stuck on the road.
The plugs only need to be changed at about 100,000 mile intervals. Your new mechanic is up on things regarding Fords and that is good, but I wouldn't rush into a plug change if it's not needed. I believe the "kit" he was talking about is a insert kit for repairing the plug hole if it strips or should blow a plug out. You might want to pick up the kit that runs about $40.00 at Sears and many parts stores just to have on hand as there have been people that have had plug problems a long way from home and had to wait for parts or the repair kit and sometimes were financially raped by a dealer or shop for the needed repair(s).
This is the kit I used;