Just a tip: all you must do to let it fit in M4 receivers is replace the air nozzle with an M4 air nozzle.
I've had this gearbox for about 5 months and there is no wear on ANYTHING. My first piston stripped on a lipo and I bought a replacement KWA piston and shaved the tooth right next to the initial catch tooth, works flawlessly now. I replaced the piston head with a systema duracon bearing piston head, but that gave terrible compression so I added on a #14 O ring from ACE hardware. (systema uses a slightly small O ring.)
Hope this helps!
NOTE: THE AIR NOZZLE WILL NOT WORK WITH REGULAR M4 HOP UPS, AND NEEDS TO BE REPLACED WITH A STANDARD M4 HOP UP BEFORE USE.
Hi torque gears
reinforced gearbox shell
8 and 9mm bushing combo
metal spring guide
aluminum non ported piston head
need to shave one tooth off the piston before lipo use
the greasing could have been better