Originally Posted by jay_bob

Get the engine warmed up. Ok to run without the manifold to get it warm, use common sense and don't let anything get sucked in.

Try AeroKroil penetrant, nothing else compares.

I would get it warm, pull the manifold, and apply the penetrant in the evening (like Friday night). Then Saturday morning start it up again and let it get heated up. Then hit the plugs again with the AeroKroil.

Wait a few minutes then use a battery operated impact to free the plugs. I have read many threads on this and this seems to be the best tactic. Don't use a gorilla 3/4 inch air compressor impact, just a little 1/4 inch battery drill type with a socket adapter. The impact has several benefits. One is that the many small shocks are better at breaking the plug loose than the constant force provided when you turn the ratchet. Also applying force with a ratchet tends to be off center due to the physics involved, this ends up applying an uneven force to the plug and I suspect this is what really ends up snapping them off.

Also don't forget to pick up a tube of Anti-Seize for the new plugs. And be sure you have either the official reamer tool or use an appropriately sized drill bit smeared with grease. Use No Power Tools on the reamer!!