This link has been taken from JSG blaster, a UK company / guy i think. Its an interesting and insightful read. One thing i have never done until now is fit the piston by itself into an empty gearbox ready for reassembly. Chuck a few screws in the gearbox shell and move the piston back and forth to check how freely it moves. He also suggests to do this with the tappet plate. I was surprised how tight some gearbox / piston combos were.