Candle wax (paraffin) is a much better penetrant for breaking loose rusty bolts in cast iron than PB Blaster, or other penetrants. Get the stud as hot as possible with a Propane or MAPP torch, and touch a candle to it. It will SMOKE and might even flame up. Once the wax has penetrated the threads, double nut the stud tightly and work it back and forth with a long breaker bar and a deep socket. Patience! It will eventually break free and unscrew without any damage.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!