Originally Posted by Parrot of Doom

They get stuck because of carbon buildup along the sides. The thread will likely be fine.

I would echo what everyone has already said. Lots and lots of penetrating oil. Also try Loctite Freeze & Release. Oh, and time. Lots of time.

Oh, and I know somebody who had the same problem. Fired the engine up and the glowplug ended up stuck in the garage wall I wouldn't recommend you do that though!