The enlosed picture shows that the rotor threads on the alternator shaft have been damaged (Stipped).

This can be caused when replacing the pulley and starting the nut on the thread with an air impact, or using a hammer when removing the pulley.

Solution:

Start the pulley nut by hand initially to avoid possible cross threading of the shaft. Then tighten to spec with air impact.

Do not use a hammer directly on the shaft when removing the pulley.