How do you properly shim a starter?

the drive gear of the starter should mesh will with the ring gear but not so much as it binds. if your old starter had any shims it is usually a good idea to put them back. if your starter is binding in the ring gear put a shim in. if it is grinding and tearing up drive gears take a shim out

Agood rule of thumb, begin with a new starter, install. If the ring gear and the drive gear sound tight, add a shim and test. However, if grinding occurs and you notice you're tearing up your ring gear, you need to bring your starter closer, this is done by adding a 1/2 shim to the outboard bolt, good luck!

AutoZone has better instructions. They say that you can push the solenoid manually with a screwdriver in a hole to see how the gears line up, and use a wire gauge to see how much clearance there is.