This is an aluminum pinion gear. Lightweight gears are popular in modern R/C racing because they offer increased performance to any drivetrain that uses a pinion gear to drive the main transmission. The TRF (Tamiya Racing Factory) team in its search for ultimate performance has specified these gears to have a special coating that result in greater durability and their teeth minimize friction loss to ensure smooth, efficient power transfer. These gears are available in sizes 31T through 39T.