How do you remove a stuck spark plug?
I rewmoved a stuck plug on a dodge van 318 fuelinjected motor by using a socket that was 1 mm to small for the plug (21mm) and a can of air for dusting off computers. I turned the can sideways and frosted the plug then pounded the socket on and used a breaker bar ever so gently about three times and presto it tuned, hard at first then I pulled of the small soket and used the regular one no problem.
I have never heard of that, quite frankly. If the threads toward the piston are damaged, they wouldn't have gone in to begin with. (It sounds like the plug is coming out part way and then meeting resistance of bad threads or something.) Did a timing belt get off and let a piston smash a plug? The plug hole can be rethreaded if it's not too bad depending on the head material. I think I'd call my buddy at the head rebuilding shop.
My father, a heavy duty mechanic, says: "clear the area, use a propane torch to heat the head around the plug, try to take it out while the plug is cooler than the head". I'd suggest keeping a fire extinguisher on hand, though. My father is a little crazier than I am :)
I continually soak it in a high quality penetrating fluid. And turn it out a little and back in a little, back out a little and back in a little. Eventually, with a little skill you might get it without breaking. You may end up pulling the threads out of the head, but you can insert a heli-coil and get your new plug back in. May end up pulling the head if it doesn't go so well.
I recommend you go out and buy some Break Away and let it soak for about 15 to 20 min. Then work it back and forth until it comes loose.
Be sure you don't try the propane torch suggestion if you've already used a penetrating oil (or spray) of any sort-- you'd *definitely* need that fire extinguisher! It would be safer to apply something cold directly to the plug if you wish to take advantage of the same principle of differential thermal expansion/contraction, but I can't vouch for this technique as I've never used it. (The metal presumably conducts heat (and cold) better than the ceramic plug, so this reverse technique is doubtful.) Personally, I vote for the repeated applications of penetrating oil (let it soak) combined with *patient* back-and-forth screwing and unscrewing as recommended above. (And in the future use an antiseize compound on the threads of the new plugs and change them more frequently!--They're cheap!)
In regards to using "anti-seize" compound, it's NOT a good idea to use this. It will reduce the friction your plug needs to torque the plug correctly (9 ft lbs. usually, check manuals). If you are unfamilliar with anti-seize compound or toque wrench usage, it is best to dry install. Hand tighten, then tighten a quarter turn after that. (aluminum heads only) As far as removing the spark plug, it's best to take it to a automotive shop. To do it yourself (depending if the spark plug is still whole or just the outer threads remaining in the head), PB Blaster is good to help "loosen" the grip of the reminant plug. You will need extractors and an extractor T-handle. Best bet though is to take it to a shop, as they are responsible for any further damages resulting from extraction. Shops are pretty darn good at this practice. Well worth the money and hardship. Appox. price range is 50-150 dollars.
First--I agree with the spray it and then rock it back and forth crowd. I'll go both ways on the anti-seize: You CAN get away without it (just don't overtighten or go too long between changes). BUT, you can also use it; just don't use too much. It does make it easier next time.
Second--JUST TO MAKE SURE--no one has mentioned/asked are you using a spark plug puller? There are specialty tools for this; they come stock w most ratchet sets. They have the rubber boot inserted to help not damage the plug and to cushion the application of force. USE ONE OF THOSE. Now for the upsell--they also have specialty tools (cheap) for getting the plug out that are like a wrench w a knuckle so you can twist better without having just a straight shot only. Specialty tools are not always necessary--but they are always helpful. You local stores have these--just ask. Take a slow walk through the tool aisle.
Heat could be the answer but I don�t recommend the blow torch idea. Try running the engine up to normal operating temperature then unscrew the plug 1 /4 turn. Re-tighten it a bit less than 1 /4 turn and then unscrew it a bit more. Repeat this until the plug is free. If the plug is simply too tight or snaps there used to be a "helicoil" kit available. Admittedly it's been a few years since I quit the trade and I don't know if they are still produced. But using these kits involved removing the head, drilling out the stuck plug, tapping a new oversize thread and inserting a "helicoil".
Some times being at a bad angle can make a big difference. If your at a bad angle with a standard ratchet try a flex head ratchet with a long handle (can also help in cramped areas) and/or possibly a long t-handle. Like everyone says above let it soak with penetrating oil like Liquid Wrench. I got mine to break loose this way. I was afraid to put too much force on it, but if you don't try hard enough you prolly won't get it/them loose. I also suggest using anti-sieze when reinstalling.