The shims are .030" thick. The package comes with 3 shims which is the maximum amount you can use.
The shims slide onto the transmission shaft that the secondary clutch sits on. This allows you to shim out the secodnary clutch to align it with the primary clutch. If you belt rides tight against the inner sheave of the primary clutch then this is what you need. Most machines only require 1 or 2 shims total if at all.