Metricnut,
I might have steered you wrong. There is no snap ring on the back of the drive gear. The item I saw on the front isn't called a snap ring, but is called a spacer. I am really reaching on this, but isn't the shaft turned down on the front, and the snout, or drive housing along with the spacer mechanically hold the armature in the starter.
In the enclosed link, you can see the reduced shaft diameter on the end that goes through the bushing. This is what holds the armature in the starter..isn't it!
I know exactly what ring and concave washer Project 39 is talking about. The starters I have at work for the Perkins diesels have this setup.
This is the picture I was looking at.
http://chevy.tocmp.com/1948_51truckshop/51ctsm1221.htm Stuart
