This is Novak's Mod 1 Steel Pinion Gear for 5mm Shaft - 16T.
These heavy-duty gears are recommended when converting 1/8-scale Nitro
off-road vehicles for brushless electric use and fit seamlessly with
Mod 1 spur gears commonly found on 1/8-scale Nitro vehicles. These
hardened gears can be used in any application and with any motor that
has a 5mm output shaft, including Novak’s HV High-Voltage Brushless
Motors that have been updated with the HV Brushless Motor 5mm Upgrade
All gears are stamped for easy identification (see additional image).
Note: These gears will not work with a 32-pitch spur gear that is used
on the E-Maxx or E-Savage trucks. They only work with Mod 1 spur gears.