There's a sneaky trick for loosening up a stubborn fitting- - - - -heat it up with a Propane torch, just as hot as you can get it, and apply a candle or a piece of paraffin sealing wax to the threads. With something as big as that cylinder head wicking the heat away there's zero chance of getting anything too hot. Once the candle wax penetrates down the thread, the fitting should back out.
Jerry
