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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.

I got a pair of Classic Army M15A1s last Sunday for £25 on the final day of Gun Ho. They sold off their rental gear at the end of the day. They have had a hard life but apart from a bit of corrosion and some scratches and cracks in the stocks they shoot all day. ANyway I have started rebuilding one of them. This one in fact. It will be for my Son Tim. He wants Magpul hardware so the stock and handguard will be replaced. ANyway I have stripped it down to it's bare components. I was surprised to see that apart from some wear on the piston teeth it was all in good nick with no slop in the plastic bushings or damage on the gears. Gearbox stripped, cleaned and laid out I am planning to replace the plastic Bushes with Steel and also plan on replacing the Piston with an alloy one with alloy head and steel teeth. I am going to put in an alloy Nozzle with an O Ring. I am also going to put in a Mosfet Any advice is very welcome.

A smaller teaser of things to come

advise to everyone, based upon your post: If you want a robust AR based AEG, get CA m15a1

Thanks for that, this is why I like this forum! Makes good sense, best failure mode is to sacrifice the cheapest component to protect the system.