Thanks to each of you for the tips/suggestions. I finally got the nut started by using a sanding wheel on the 4 in. grinder to bevel the end of the pinion shaft back into the good threads and then a thread file to bring up the tip of the threads that were flattened and finally a 3 corner file to dress up the 'entry ramp' thread. The pinion shaft is hard material as Stuart noted. The new nut was the right self-locking type that is punched to give the threads a not-quite-round profile so it won't back off.
Thanks again for the help,
Jerry
