THis: Airsoft AEG G&G GR16 M16 Receiver Disassembly Take Down by AirSplat - YouTube
Will take you down to the gearbox, fyi, The middle pin though is a b*tch, took me 2 hours to knock it out.
Will show you how to replace any part you need to in your gearbox.
IF you have trouble, or dont feel as if you can do it, contact a gun doc in your area: http://www.airsoftcanada.com/showthr...hlight=Gun+doc
In this thread.
Some good brands are,
Systema? (mixed reports)
Airsoft Research Syndicate (ARS)
Keep in mind Polycarb piston heads are probably your best bet apposed to metal ones.
Try to shop in Canada, helps out the retailers here