This is a replacement Arrma 13T BLX 4X4 HD Input Gear, an ideal replacement for your BLX and Mega 4x4 BigRock, Granite, Senton and Typhon kit supplied parts.
 
Features:
Manufactured from tough, durable materials for a long service life
Precise dimensions for excellent fit and smooth operation
Hybrid construction combines a composite gear molded on to a metal splined shaft
13 tooth bevel gear
1.35 Module tooth profile
Note: To be used with ARA310956 Diff set