Gen 9 Gearbox 2 issues

Hi guys.

I have 3 issues on my Gen 9 gearbox J10 ACR by JM

  1. Safety not working.

I suspect it might be the spring on the selector plate. Has anyone faced this issue, that the spring is too slack, and not engaging the cutoff lever properly?


On another note, when i manually pressed the cutoff lever, and the safety catch moves upwards. But on inspection, it does not appear to perform any form of mechanical safety function on the trigger. Can someone explain how does the safety catch stops the trigger from moving in safe mode?

  1. I reinstalled the motor in the ACR J10. However, upon feeding with 11.1V battery, there is no plunger movement at all, only a motor spinning high pitch sound. I upgraded to a 1.25 UEQ spring but i think that is no issue for the stock motor. Could it be a stripped gear? although on inspection, the gears appear to have no visible damage.

Another reason is motor height? When seating the motor, do you guys press the motor all the way into the gearbox from the pistol grip? or do you simply let the motor closing latch on the pistol grip press onto the motor and adjust using the screw?

  1. How do you position the fire selector disc with respect to the select switch when reinstalling the gearbox?


1: safety - i’ve heard this a couple times lately

  • so the mechanism occurs on the other side of the box inside to the selector plate.
  • there is a small leg which kicks up and blocks the travel of the trigger top forward
  • seems yours may not be blocking which could be a couple of things, sizing is most likely
  • a fix would be to build up either the front of the trigger or the mechanism

2: motor spinning high pitch sound - smells like motor height

  • make sure its not your mag motor you’re hearing
  • could maybe be the motor terminals have vibrated loose - happens quite a bit
  • setting the motor height is best explained with a shimming video but long short is
  • you want to set the bevel gear and motor height so that the full pinion has contact with the bevel but not so much that it presses into the bevel

3: position the fire selector disc - i don’t. the box does this on installation

  • well…the selector plate moves to where it needs to be to accomodate where the fire selector is