If the hole is actually stripped, which is very common in aluminum heads, there is a way to do this with the head mounted AND keep the crap from falling into the motor.
If it's carbed, the carb has to come off... EFI it doesn't matter. Carb has to come off or you'll blow fuel everywhere. Very bad. If you have EFI with Mass air, then disconnect the Mass air from the intake. You don't want to risk damaging the sensor. (Technically you could do this with the exhaust but you risk blowing crap up the exhaust into the engine. Intakes are usually pretty clean except for some carbon.)
Remove the plug.
Connect a shop vac so it blows air into the intake manifold. It doesn't have to be a perfect seal. just good enough to make most of the air go into the motor.
Rotate the motor so the intake valve opens on the cylinder you need to work. This will let the shop vac blow air out the plug hole.
Now work the plug hole over and install the Helicoil kit according to directions. Wear goggles because the shop vac will be spitting things at you.
WARNING! Thread inserts are setup by both diameter and the lenght of the threeded section of a plug. You must install the right lenght sleeve as well as the right diameter. Helicoil brand sets usually come with one of each length in the kit.
11-Sept-01, The day the world as we knew it ended.