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you can try a thread cleaner as suggested..or get the head heeli-coiled..
you can buy kits but i wqould STRONGLY advise you to get it done by a pro..
basically they expand the hole and then cut it with a bigger thread..then screw in a insert that has the right thread for the plug in it...the TRICK is to get it straight and not to drop bits into the cylinder while doing it...
i have had it done on vehicles before...personnaly i have always taken the head off and taken it to have it done..but i believe it can be done on the vehicle...
if you take the head off its a great time to also decoke the block, fit a new gasket etc...
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