You should note this, DO NOT FIRE YOUR GUN WITH THE BOLT BACK. There is a design flaw with the ARES G36 (EBB) where if you fire the gun with the bolt back it will shred your gears.
As for replacement; VFC has some nice "auto shimming" gears with springs on them that will do all the shimming for you. Get this set if you want, it should be compatible but I can't guarantee 100% if fitment will be 100% right due to tolerances of different brands.
http://airsoftgear.ca/index.php?main...roducts_id=957
Otherwise Element actually makes a pretty kickass "economy" gearset. I have a set of Element Torque gears in my gun and it shoots pretty well.
Modify also has a "modular" gearset that is a different take on the "auto shimming"/preshimming gears but the danger is that if you break one gear the whole unit is toast or something.
http://www.airsoftparts.ca/store2/in...oducts_id=1226