This guide will take you through the process of assembling a version 2 / 3 gearbox. In this guide I use a version 2 standard gearbox from an MP5, but most of the guide is the same for all types of version 2 and 3 gearboxes. And principles also work for allot of other versions. But be aware that there are a lot of different parts out there, so if your gearbox looks different it's properly just another manufacture, but all in all probably the same part. Note that I DON'T take any responsibility!
The bottom gearbox side placed with the outside up.
Add the fire selector plate. It slides in from the front.
Add the safety selector pin. preventing the fire selector to slide out.
Turn it over.
Add the single shot grip arm and screw it down. Make sure it can move freely.
Turn the box over and add the small selector spring between the small arm and the fire selector.
Add the trigger switch (with wires) and screw it down.
Now it looks like this. Route the wires down to the motor holes in the bottom, and out the back or front if the battery is in the front of your gun.
Add the trigger switch pullback spring.
Put in the safety spring to the safety leaver going throe the box.