Now take the cylinder set the cylinder head in side if it isn't already. The placement should be such that the two parts fit flush with indentations in the gearbox. This done to check if this is the case before you put the other parts on the cylinder.

Now take the cylinder out so you can add parts to it.

Then slide the air-nozzle on to the cylinder head and slide tappet plate onto the air-nozzle.

You can now put cylinder and all the parts attached into the gearbox as shown.

Now put the piston in the cylinder. Make sure the piston tacks aligned properly with the track of the gearbox and the teeth of the piston are facing the gears.

Finally take the tappet-spring and attach it to the tappet plate hook and the gearbox post shown. Without this spring the gun will not be able to load BB's.