Strengthened and notched gear shafts, spare part for the HPI #87193 4 Bevel Gear Differential Conversion Set for cars or trucks. The #87193 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 drive train for longer gear life and less maintenance.