gordo2472003 said:
the welding a nut on is a very good solution
but you need warm it up with blows thats what it called in nblacksmith talk "warming it up"
beat on it, that is the stub
now i dont mean kill it what you will be trying to do is jar and impact the threads by tap tap tapping the protruding piece
light to medium taps all clock directions,and straight shots on the end with a 1/2 pounder and a flat nosed punch if you cant access easy
if you are mushrooming the stud alot you hit too hard boy
hit it less hard in the weak direction if you aint got a good feel for metal ,or git mad and the red curtain falls,you can break out the casting in the weak direction the port side usually
this will work the threads compacting rust or whatever and just make them loosen up
then you want to get some heat on the surrounding aluminum
put alittle penetrant on the threads and when it starts smoking real good that is 400F or so then it shouild come out clean just gripping it
be patient with it and dont hit it too hard and dont hit it sideways when hot
thats all