If the shims you are talking about are between the head and the liner, they are there to set the compression ratio to suit different types of fuel. My Picco 26 has 3 shims and is designed to run on 25% nitro. If I were to drop to 10% nitro then I would remove 1 0.1mm shim and if I were to go up to 30% nitro then I would fit an extra 0.1mm shim. The amount of shimming and thickness is part of the engine design but the basic rule is "More nitro, more shims. Less nitro, less shims"

Hope this helps.