...many moons ago when I was turning wrenches, KD used to make a combination reamer and tap for installing steel thread sleeves in damaged aluminum plug holes....these were much better than a heli-coil, since they were solid sleeves.....the tool oversized the plug hole (of course you had to be sure that the piston was down) and cut new threads....once the newly threaded hole was cleaned, the sleeve was installed on the plug (grease on the plug threads and loctite on the sleeve's outer threads).....I have no idea if KD is still in business, but you might check with an old import car mechanic and see if you can find the tool and correct sleeves...
