When you place the 12 teeth gear at the transmission axle, do not tighten it against its ball bearings. Leave a bit of clearance, between 0 and 0,5mm, so that the axle has a bit of axial movement. You will reduce the temperature of the functioning of the transmission, softening its working and endurance the bearings’ life and the plastic big gear.
Once the gear is placed, tighten it strongly placing medium screw locker (blue Loctite 243 or similar) at its bolts without head.

This axial movement can also be searched for in the differential socket, achieving thus identical benefits.