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This article is in video format at the bottom of the page! How severely can a spark plug hole in an aluminum engine head, which previously had a repair insert and has since blown out, making the hole bigger and showing signs of erosion, be damaged and still be fixable? Basically, there needs to be enough material to ream or drill out so that complete threads can be tapped to accept an oversize spark plug insert. The image above shows a spark plug hole after a Napa Fix-A-Thread insert failed, lea …