Reassembling a gearbox could be hard for some especially those who are new to airsoft. I am posting a step-by-step reassembly process with tips on how to make it easier.
Parts that are totally removed
First step is to place the gears as shown above.
Then place the cylinder together with the tappet plate, cylinder head and nozzle, attach the spring at this point
Then place the trigger, sometimes this will not stay in place, try to use some grease to keep it in place.
Then insert the piston carefully not to dislodge the tappet plate.
The place the anti-reversal latch, sometime this won't stay stay in place the bevel gear
Now, using a screwdriver inside the spring guide, push the spring guide and the spring in place.
Now replace the cover carefully not dislodging all the spring things inside.
Install in the lower receiver
Put back the grip and the motor.