Removing/Replacing Pulley Nut -- School Bus Alternator

[a64pilot]

Hand impact is great for screws etc., not likely here, plus to use one you smack it with a hammer, that force may not be good for the alternator.
When it's replaced my opinion is this is a good use for blue Loctite

[donradcliffe]

I just took the alternator, the right size socket for the nut, and a $10 bill into the nearest auto mechanic. Their pneumatic impact wrench had it off in less than a minute.

[Rorzech]

Usually they have a hole for a big Allen key in the front end of the shaft to hold it for the wrench. Good luck.

[UNCIVILIZED]

Impact wrench = bad for the delicate bits that the nut's connected to. Maybe a Dremel or a hacksaw? And cut 90% of the way down to the threads on 2 or 3 sides, & then split the sections apart by prying them with a screwdriver.
Or if you take the unit off, & put it on a work bench, after making your precise cuts, support the nut's underside & the shaft. Then gently tap a cold chisel into the cut on top of it to split it apart.

Jewelers files are handy for delicate cutting too, along with of course thread repair. And so are a few other metal working tools designed for finer work.

That said, almost regardless of the tools used, the usual methods for loosening it up first should be applied. Penetrating oil, heat, cold, etc.