Well count me out of the picture here, you obviously are running into problems that I’ve never faced. I don't know what you have there. I don't have a 12v generator here to compare to, only a few 6v ones and those measurements I posted earlier were what I found on the 6v generators that I have on the bench.
If you are measuring the shaft at .670" then indeed you’re measuring a 17mm diameter (.6693”). However, like I said earlier, M17x1.00 or M17x1.5 are the only thread combinations that are used as far as I know and M17 nuts would be special nuts from any supplier and not a common size. If you have access to a thread gauge (most full tap & die kits) it might help to know the pitch that you have on that armature. I just can't imagine GM using metric specs back in the day.
Don't try using a bad or good nut to chase the threads on the armature or your going to have both of them bunged up. If you read the old post I mentioned a thread file for an external thread, they're relatively inexpensive and I wouldn't be without a set.
I’m sure that Ken Warner at Classic Generator could send straighten you out on this matter and send you the proper nut for the generator that you’re working on. He has always been very helpful whenever I had a question or needed parts and is a regular contributor to the forum discussions.
http://www.classicgenerator.com/ Denny Graham
Sandwich, IL
