I'm sorry, but I think some of the information above is not exactly correct.
First, I suggest 'choosing your battles'. If this is not a fight you have to do, DON'T. After all, it's getting to the point a glow plug (gp) isn't needed until next winter.
If you get it to loosen, but are unable to remove it, and THEN break it off, with the plug off it's seat, you will be removing your cylinder head. We have drilled out hundreds of glow plugs. We are good at it, but I would rather never do the next one.
Here are the ground rules for the best way to remove a stuck glow plug:
First, get a very accurate torque wrench and make sure exactly where 30 ft lbs of torque is. We have a simple and accurate method for properly setting a torque wrench.
A clic-stop torque wrench is perfect. The point is, when the torque applied on a glow plug is 33 ft lbs, that is all the thin wall between the 10mm nut on the g.p. and the body of the plug can take. Any torque above 33 ft lbs and you can expect the nut to shear and then, all it will do is spin in place. So, methods like a Tee handle or a 'Burst of energy'... how you going to guess at 30 ft lb 'burst'? However, I will agree, attempts with the engine hot will promote removal, as the aluminum of the head will expand more than the gp when heated.
If the gp will not budge, and the nut shears, the only choice then is to break off the nut and with the cylinder head on the bench, drill it out. More on the drill-out to follow...
As long as the gp is intact, we suggest a very good penetrating oil and to build a clay dike in front and around the gp so each time you come to a stop for the day, you can pour a 1/2 teaspoon of penetrant around the gp and let it work it's way into the threads. We may do this for a week before actually attempting the put torque on the gp.
One of the rules is, if the torque wrench clics at 30 ft lbs, STOP! Go the OTHER WAY! It doesn't matter if you loosen or tighten the gp, just get it to move. Once I am able to get it to move even the slightest amount, I can get it out, but don't pass 30 ft lbs torque. It may only move 1/4 turn and stop. Retighten it and loosen, tighten, loosen... eventually, it will move a bit more. I'd rather spend 45 minutes taking out a gp, than 90 minutes drilling one out with the cylinder head on the bench.
If you are able to remove the gp, it's not 'all done'. You need to check the threads on the gp and the threads in the hole. If you gp is damaged or stripped, so also are your threads in the bore. We use a 10 x 1.0 tap and a tap Tee handle with a 1/4" x 6" ratchet extension in it. Turn the extension 'backward' to how you would normally use it, with the square of the tap going into the female end of the extension and the male end going into the Tee handle. This will help you get far enough away from the head that you can spin the handle without hitting the cylinder head. You may find it helpful to use an arbor whose I.D matches the diameter of the tap and rest the arbor on the shoulder around the gp hole. That surface is square to the gp hole. Gently, by hand, turn the tap into the hole, attempting to follow the original gp threads.
The most common stripped gp hole is the #3. The reason is that the vacuum ball gets in the way of a straight shot at the gp threads. Remove the two bolts or bend the bracket holding the vacuum ball and get it out of the way when removing or installing gp's.
If the gp hole either has stripped out threads or damaged and pulled threads, we recommend using our Glow Plug Recoil Repair Kit. Although some feel the TimeSert are a better insert, the Recoil repair is simpler and cheaper. Also, in the event the Recoil does not work, you can still use the TimeSert as a 'second attempt'. We have LOTS of recoils installed and almost zero failures over at least 10 years. We have proven this methods effectiveness.
It may be the threads are good, but you may see is a bunch of carbon packed up into the threads of the gp or the gp hole. This is caused by inappropriate seating of the gp contact bevel to the seat inside of the cylinder head. If the carbon built up on the seat is not removed, the gp will seat on the CARBON instead of the aluminum of the head. This will cause the gp to leak slightly and carbon from the combustion chamber will force it's way into the threads, making a very hard to remove gp. The angle of the seat, if I remember correctly, is about 50 degrees. We make a gp seat cutter designed to quickly and safely remove the carbon and recut the gp cylinder seat for the proper contact surface and angle. Some have used an old gp and ground the threads off of the body so it will spin in the bore, then cut a small slice into the seat bevel on the gp. This can also remove the carbon. If you can't see a clean circle of aluminum at the bottom of the gp, you cannot expect the gp to seat without leaking.
Now, if the glow plug breaks off, you have few choices. The most important thing to know is the gp is at a COMPOUND ANGLE. It not only tilts away from the head at a 60 degree angle, but also 15 degrees to the side. There are many ruined heads that the drill went through the side of the gp into the water jacket, ruining the head. We use a drill in the gp hole to find the proper angle to drill. For example, if #3 gp needs drilled out, we put a 17/64" (I believe that is the best fit..) into the #4 gp hole, then tilt the drill to match the angle for drilling out #3. Methods...1) Use hand drill and gradually increase the hole size, making sure you do not drill down to the gp seat. Finish with a left hand drill bit, which will spin the shell of threads out of the hole. 2) There are machine shops that can remove the gp with an EDM machine, which basically, burns the gp out of the head with an electrical discharge. Figure $100-$150. Or, you can send it to me... I usually get them out for $90.
Another common mistake is expecting an easy-out to remove the gp. What we end up getting is a gp with an easy-out broken off in the hole, which compounds the effort to remove it. DO NOT use an easy out. I have NEVER seen it work.