In the last 45+ years of wrenching professionally, I've had my share of seized plugs. I started my career on air cooled VWs, aluminum heads, with 1/2" reach plugs! Talk about paranoid about stripping threads!
Then came Japanese imports, with aluminum heads.
I've found that it's not ONE technique that works on all of them. Experience does give you a "feel" for how much force is too much, hard to put that in words.
I generally tackle plug changes as soon as I can work on the engine without burning myself, need to get a hustle on to get the jobs out. That being said, if I come across a tight one, I will give it a go at that time, but if it's a stinker I'll leave it to last.
If the plug comes out part way and tightens up, you have a chance. You can shoot your favorite penetrating spray around the base to lubricate what threads are exposed. I use Moovit. Give it a few minutes and then wind it in, then out, then in, then out. Each time going a wee bit farther out, adding more penetrating spray as you go. This works in the majority of cases. If the plug gets tight again, and this can happen with the plug not seated, you will want to run the engine and try it again hot. If no go, then spray some cold water on the plug and try it quickly before it warms up again with the engine heat. If still no go, let it cool down and try again. The hot and cold cycles can work for you.
If the plug is really tight, as in your case, I will
ONLY use a 1/2" drive. Using a 3/8" ratchet, or bar, and an extension creates too much springiness. Even with a cheater pipe the springy tools seem to absorb the torque. This applies to other case of a tight fastener with a too light tool, like a head bolt or shock bolt.
Use a 1/2" drive extension, the shortest that will work, and a 1/2" drive breaker bar. If you don't have a 1/2" drive plug socket use a 1/2" to 3/8" adapter to the socket.
Tug on the end of the breaker bar WHILE supporting the joint over the extension so it doesn't tilt on you. You will loose effectiveness if it tilts and it may break the plug.
This is where experience comes into play. How much force is too much? Well, when the plug has snapped off in the head, is was too much!
The plug and threads will stand up to a fair bit of force before you damage something. I apply a steady force, slowly increasing. I can't say how much, I'm not that strong but I'd say a good 150 ft/lbs
If that doesn't work, then run the engine hot and retry. Do not drain coolant or do anything to make it run hotter!
Try now that it's hot.
If this doesn't work, let it cool down till it's cold and try again.
If this still doesn't work, you have two options: keep increasing the force until it either comes loose or snaps off or strips the threads. Or, you can pull the head and take it to a machine shop.
If you try harder and it comes free, you escape the re&re of the head. If it snaps off, you haven't lost anything.
If it does come loose, you need to "feel" as to how it's coming out. At this point you are already ahead! A plug that is moving trumps a seized one
If the plug is unscrewing easier and easier that may be a good thing, or, it could mean that it has already torn the threads out! Either way, you will deal with it.
If the plug starts to tighten up, get some spray in there and work the plug back and forth like I said earlier. Spray is always good, always good to work the plug back and forth.
So now, hopefully you have the plug in your hand.
You need to examine the plug threads for traces of aluminum transfer from the head. If nothing, you are looking good! If you see some aluminum, you need to run a thread chaser through the plug hole before you attempt to install another plug.
If at ANYTIME you are installing a spark plug and it won't go in more than a few turns, you MUST run a thread chaser through the hole.
NOTE: you can ruin all your efforts if you start the thread chaser crooked!!! Ensure that the thread chaser is straight up! Start is will a little effort and work it back and forth as you tilt it a wee bit to get a feel for where it seems to want to go in easy. Once you believe that it is straight, SLOWLY, CAREFULLY increase the force to get the thread chaser through until you have gone in several turns.
BE SURE YOU HAVE THE PISTON DOWN!!!! You don't want to screw the thread chaser into the top of the piston!
Piston down, intake valve CLOSED.
Use some lubricating spray on the thread chaser once you are sure it's going in straight. Work the thread chase back and forth until it passes smoothly.
Now take your shop air, with a 3/16" brake line tube screwed in, and stick it down the plug hole and blow that chips out, watch your eyes!
ALWAYS, ALWAYS use an anti-seize compound on spark plug threads. Not just on aluminum heads, ALL heads. With the long life plugs used in today's engines, these plugs may be in there over a decade!
In the end, you may still have to pull the head. You can only do your best. Good luck.
