Having been through a few nightmares with a starter, my suggestion is this:
The best way to overhaul the starter is to take the old one out, and exchange for a Bosch factory-remanufactured one.
In the long run it might be the easiest and least-costly method, and certainly will involve the least amount of time.
If you are overhauling a starter my guess is there are other things on the "to-do" list that take time and money, and not all of them can be as easy as an exchange for a factory remanufactured unit.
You can probably exhange at the CC, at a dealer, or at many Bosch authorized service centers around the globe.