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This issue has been around for 40 years or so on any engine with an aluminum head. It is caused by dissimilar metals corrosion and any mechanic worth his salt knows that when you change spark plugs, you must coat the threads of the new plug with something . . . . usually an anti-seize compound specifically made for spark plugs which keeps the corrosion from 'eating' the least noble of the metals . . . . the aluminum in this case Recently (the past several years) NGK, Delco and possibly some others have begun coating their plug threads with a trivalent plating (looks like chrome) specifically to reduce the corrosion - If your new plugs have the coating, don't use any other compund on them too - The two together act like a lubricant and will cause you to torque them too tight As always, with an aluminum head, be SURE to use a torque wrench when installing plugs and err on the side of sliightly under torquing them, rather than over torquing them - Just enough to crush the washer is all that's needed. The aluminum is soft, the threads are pretty fine and it's not that hard to strip the threads, especially in an older engine which already has corrosion present Don