This TT-02 chassis option part is a gear set featuring a newly-designed aluminum spur gear mount and spacer. It enables gear ratio settings from 9.82:1 to 6.55:1, and it can also be used in conjunction with a separately-sold 64T spur gear for lower gear ratios.

TT-02 spur gear-mount x1
7x5x11.4mm spacer x1
FF-03 PRO spur gear (68T) x1
2x9mm stainless steel shaft x1