Slide the air nozzle over the cylinder head and attach the tappet plate to the air nozzle. Hook the tappet plate spring back onto the tappet plate and the post in the gearbox. Place the piston in the cylinder.
Press on the air nozzle to ensure that it slides freely and springs back into place.
Carefully place the piston spring and spring guide back in place. Ensure that the spring guide is locked into the slots in the gearbox, and that the piston is properly seated in the rails.