Description: Modify Advanced Shims Shims are one of the smallest pieces in the gearbox, but they are also the most important. Shims are responsible for spacing the gears properly to avoid uneven and premature wear. An improperly shimmed gearbox is doomed to failure. In order to shim a gearbox precisely you need precision shims, and Modify has just that. These shims are precision cut for exact spacing, with these shims, the only thing else you need is precision tech to do the work.