Now you can add the spring and spring-guide.

Make sure to hold the piston down as shown wile working further.

Now make sure that all of the parts are not out of alignment or set in a way that would stop the gearbox shell from meeting.

The most common offenders are the safety latch, the anti-reversal latch, the rear wiring and the piston.

You can now take a long thin screw driver and slide it threw the hallow spring guide to hold down the piston and take the other half of the gearbox shell and line it up first with the front of the gearbox and begin pressing it in place.