I think in most cases they work themselves loose and pop out of the head, damaging the threads in the process. With the aluminum head, the problem is maginified. It used to be Ford installed a Timesert, but want to replace the entire head. There are shops that are able to install Timeserts or similar and install a new plug. It's not super common, but does happen, so snug the plugs down when you get it or just replace them (a better idea since you have to remove the COPs anyway).
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Originally Posted by HardDrive
What the problem with the plugs? What is 'not doing them right'?
I've changed every plug, on every vehicle I've ever owned the same way. Finger tighten them down, then 1/8 turn with a wrench. Hasn't failed me yet.
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