Need a little more info here. Did it just start happening?, or has been an ongoing problem? You're talking about shims so I would think you're installing a new/different starter. In over 40 years of messing with this stuff I have never once had to shim a chevy starter. One key factor is the battery power. It must be a good hot battery, with plenty of CCAs. Another important piece is the brace that comes off the brush end and attaches to the block. Those 2 items, the battery and the brace will solve your problem, assuming you have the right starter nose for you application................MP