[Veselin Kolev]

I agree that only carbide tooling should be used.

I am guessing that you want to press fit a .3125" shaft into a hole in the sun gear. Generally you would be fine with a good thousandth of an inch under that, or a .3115" hole. I think if you get a solid carbide reamer that is .3115" and run it really slow through the hardened steel, you'll be fine. Remember also that reamers tend to chatter a lot if you're going fast, and when it chatters you kill the precision and damage the cutting edges. As long as the reamer is running cleanly and not chattering, it should give you really close tolerance, especially with carbide tooling.

Good luck.