This is an optional 4 Bevel Gear Differential Conversion Set for the HPI Savage 25 RTR, and Savage SS monster trucks. This is also the same gear set that is included as stock equipment with the Savage Flux. The 4 gear conversion is a popular upgrade, especially for heavy-duty driving and high horsepower applications. Now HPI offers a precision factory manufactured set that is a direct replacement for the stock gears. The extra gears decrease the stress on the drivetrain for longer gear life and less maintenance. Each package contains everything needed to upgrade one differential, including strengthened & notched gear shafts, and revised gears that are hardened for extra durability. This is the stock 4 bevel gear differential for the HPI Hellfire, Savage LE, Savage X and Savage 4.6 monster truck.