Stripping the threads in an aluminum head is pretty easy if you aren't careful. It's also pretty hard if you are careful. The trick from Haynes: Use a piece of rubber hose on the end of the plug to start the threads. If you aren't in right, the hose will slip. I personally use VERY slight touch from my fingers on the ratchet extension. If I get ANY resistance, I pull it out and start over.
I stripped a spark plug in my old Isuzu Trooper I-4. Pulled the head to put the Heli-coil in (was afraid of shavings in the cylinder, I hear some grease on the tap will catch them). Found a burnt valve. Sold the truck to get the SVT. Poor dealer took it to a mech I know to get it checked out ("That tick is just a little exhaust leak" is what I told him). The mech laughed and told him he knew the truck and he was looking at $$$. I actually saw it on the road 9 months later, ticking away (sucker).
