Recommended shim by Vex in another thread:
If you notice your Falcon doesn’t have a shaft shim, there are two options:
You can contact VEX (link above) and we can send you shaft shims free of charge.
You can buy a box of 25 shims from McMaster-Carr. The part number is 94773A712.
Note the recommended shim is .020" not 1/16"