After some research, I discovered that this is a common problem for the modular engine family. Look closely at the threads in this picture. I brushed antiseize onto them prior to screwing the plug into the cylinder head. Notice how only the bottom five threads have been exposed? The rest of the threads are untouched. There are only five threads holding the spark plugs into the heads, and I’ve seen early, non-power-improved heads with only three spark plug threads. Supposedly, the 2004 and later versions of the 4.6 and 5.4 SOHC engines have seven threads. There is some debate online about whether you should use antiseize on new plugs when installing them. I do and will continue to do so because of modern spark plugs’ long change intervals. I’d rather have to retorque my spark plugs periodically than have them freeze up in the cylinder head. The spec in my factory service manual is 13 ft-lb.