[mentorDon]

For a light press fit, a little light machine oil should be used. This helps to prevent galling between the surfaces as they slide together. Heating them until red hot and letting them cool down without quenching would soften/normalize any previous heat treatment. There are 'air hardening' tool steels, but it properties wouldn't be useful for gears.

Also make sure you are pressing against the opposite end of the motor shaft and not the motor body.