KROIL or the home brew ATF/Acetone mix will free up that shaft better than any of the other penetrants you can use. Liberal use of a Propane torch also works well, and the chances of damaging a chunk of cast iron that's routinely exposed to exhaust heat is somewhere between slim and none. I've seen exhaust manifolds on motorhomes glowing cherry red on a long uphill pull, many times!
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!