the problem is that the plug has stripped out the threads of the head,,,,,most ford dealers will want to replace the heads because of this,,but,you can fix it yourself with a spark plug repair kit that you can buy at your local parts store..it consist of a heli coil thread kit and a threader ..or,,you may be able to get an independent shop to do the repair for you....
you also need to replace the plug boots on the coils if they have never been replaced,,these will jump a spark to the head when they go bad
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