Impeller Shims (3) total, used for fine tuning your pump set up to allow the impeller to move away from the stator blades. This adds additional bottom end slip, causing the motor to spool up more creating more low end hit if your impeller is slightly pitched too high.
Shim kit contains:
(2) 1mm shims
(1) .5mm shim
For Yamaha based 20mm shaft applications only.