If the clamp is too tight, it ever so slightly distorts the the rack which affects the internal workings. If you've ever had one apart you'd see the very close tolerances inside. Shimming the clamp reduces the pressure on the rack and lets it relax back into it's allowable internal tolerances. The need for shimming is intentional due to possible differences in those tolerances. The shims should be included in this kit, look like large washers cut in half. Some trucks need none, some need on top or bottom, some need one top and bottom, then there are trucks like mine that needed both shims top and bottom.