Stick with plastic teeth on the piston and a plastic nozzle. Metal teeth on a piston are normally tougher than the gears so if something goes wrong the piston ends up destroying the gears and not the other way round... Pistons are cheap, gearsets aren't!
Plastic nozzle is for a similar reason, if something goes wrong the nozzle will smash your hop unit to bits if it's metal, you'll also run the risk of chopping BBs up occasionally.