Does anyone have the original and minimum diameter of the commutator for 57 Tbird generator? 

This information probably will be of little help but we went  by just eyeballing the thickness of the copper and then determine wether the commutator could be turned one more time.   The shop manual only says remove no more copper than necessary to clean it up and undercut the mica 1/32".  Sometimes the parts catalogs can be of help but not in this case.  Generally speaking, a Ford generator commutator can be turned several times to clean up "normal" wear. Send a couple pics of the rear end showing the copper thickness and a side view showing the surface condition. After viewing the pics, a determination could possibly? be made wether the com could be cleaned up, if that's what you are wanting to do. The generator in question is probably the common 30 amp generator so hopefully someone can find dimensions for that one. 

64F100 57FAIRLANE500

Thanks for the reply.
I've turned it down now and did the undercutting. It looks ok and the brushes sit fine on it. I couldn't find the correct brushes here in Australia. My local auto electrician gave me some brushes he had laying around that where a bit wide. I filed them to size, so that should work ok. The tracked shipping makes things ridiculously expensive for a cheap part. The parts suppliers won't put them in an envelope and do standard $1.70 to Aus post.
I finally worked out why the bearings were measuring up strange sizes. It has metric bearings. Which leads me to believe it is a reproduction generator. Has anyone else come across this?
 

You did all the right things with the commutator - sounds very good. Just make sure the brushes slide easily within the holders and the shunts are firmly attached. As far as bearings and sizing, they are expressed in metric terms. Trying to remember?? the drive end bearing I think is 6203 with end seals and it is expressed in metric terms. 

64F100 57FAIRLANE500

Wow! I'm very surprised bearings with metric dimensions were being use in the USA in the 50's. You learn something new every day. LOL

The generator is charging again.
Voltage was a bit high, so I had to adjust the regulator.
The job was more work than it should have been. Sometime in the past the rear bolt was put in from the block side and it would have been bolted on with the bracket to the timing cover. So I had to remove the mounting bracket to get it off. That's when I discovered the bolts go through to the water jacket. 😒So drain the coolant and clean up off the floor. LOL
Not sure why Ford made it so tight against the block and head. Quite a squeeze to get a nut on the bolt when I put the gen back on. I had to tape the nut to an open-end spanner/wrench to hold it while turning the bold into the nut. Even the rear terminal will hit the head if you try to swivel the generator up.