This is a Hot Racing 57T Arrma 4x4 BLX Steel 0.8MOD Spur Gear, an optional steel Spur Gear for many Arrma vehicles.

Features:
Rugged 32 pitch (0.8 module) for total reliability when using high power brushless systems
CNC machined and lightened hardened steel construction
Replaces factory plastic gear
Black oxide coating for durability and corrosion resistance
Can uses factory slipper pads (AR310790) (AR310908) or Hot Racing ATF15H ATF15HN aluminum slipper pad(s)
Specifications:
Number of Teeth: 57
Gear Pitch: 32 pitch (0.8 module)
Tooth Face width: 0.24 inch (6mm)

Compatible with:
Big Rock 4X4 3S BLX: AR102711
Granite 4X4 3S BLX: AR102666
Senton 4X4 3S BLX: AR102668
Typhon 4X4 3S BLX: AR102696