I messed up my alternator.
 

Took the housing and the fan apart to detail and paint, got a little impatient during disassembly, tapped the alternator shaft to seperate it from the fan with a hammer- I now have 2-3 threads deep messed up.
Can I grind off those top threads and still have enough space to properly seat the fan nut? Does anyone know about alternator internals? Am I better off disassembling and having a new shaft machined or do I bite the bullet and replace the entire alt? Anyone got one laying around? 1980SC w/ external reg.

Take it to a machine shop. Or, you can sacrifice a die by cutting it in half with a cutting disc and clamping it on the good threads then running it out over the damaged threads. Not too tight, you just want to be able to run a whole die back over all the threads. So, that's 2 dies. If you can get them cheap, it'll work. There is also a tool called a thread file which will get you to the point of being able to use a whole die. It takes some practice to use one and you have to be sort of mechanically inclined.

You should always leave the nut on out at the end so that doesnt happen. I would use a small file and a dremel tool to get the threads and the end nice enough to run a die over it

there is a thread saver on the griotsgarage website. I have never used it so I don't know how well it works. I'd say any of these approaches is a one time only shot. Once the threads are too far gone, I'd personally think about tapping it for an SAE nut. Why pay for a rebuilt alternator?

well thats what I figured, Id rather not buy a new one, Im taking it to the machine shop tomorrow morning, because I do have more faith in the gentleman there than in myself to get it right the first time...

And yeah, definately lesson learned about leaving the bolt on to tap on than on the ends of the threads themselves...

I ended up doing the very same thing as part of one of my first DIY projects. The good news is that there is thread restorer file. It looks like a cheese grater for lack of better explanation. See here for example: http://www.sears.com/shc/s/p_10153_12605_00999044000P?vName=Tools&cName=HandT ools,GeneralPurpose&sName=Bolt-Out,%20Taps%20&%20Dies&psid=FROOGLE01&sid=IDx20070 921x00003a Align it with a couple of good threada and file away the compressed threads. It worked on mine like a charm even though it looked pretty trashed at first.

Ingo

"Thread files" work great for restoring the smooth thread pattern of many different sizes of threaded items. Just make sure that you get the proper size file. They normally have 4 different pitch and thread counts available on each end of the files. Here are some from fleabay:
http://cgi.ebay.com/Thread-restorer-file-set-SAE-coarse-SAE-fine-metric_W0QQitemZ120266285686QQihZ002QQcategoryZ207 62QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Since the mechanical integrity of the threads will be compromised by filing, do you need to use some kind of thread lock product? if so which one? Can all this be trusted @ high RPM?:confused:

not really - typically only the first couple of threads are affected. Filing them allows you to get the nut started. It will go past the filed part so there should not be any issue