This is an optional Arrma 57T HD Mod 0.8 Slipper Set, pre-built and ready to install in your BLX 4x4 BigRock, Granite, Senton and Typhon vehicles.
 
Features:
Precision manufactured aluminum slipper hubs
Durable composite 57T 0.8 module spur gear for reliable power handling capability
Castellated slipper pads for assured location into spur gear
Stainless steel slipper plate
Supplied assembled and set, ready to install into your vehicle