I had exactly this same problem with a magma grill. PB Blaster, all manner of tools, let it soak for days, hammer, vice grips, you name it -- finally heated it red hot with a propane torch. I heard a nice little "tink!" and sure enough, loosened up and came out nice and easy. Makes no sense to me, a grill gets hot all the time, but as the previous poster says, that propane torch did the trick.