UA_Gearbox-V2 Guide

Before proceeding, make sure you have the right tools readily available and contact us with any questions you may have at


Tools needed

  • Micro screwdriver kit
  • Long Philips (PH2) screwdriver, >6.5 inch in length
  • Punch set (big enough for receiver body pins and small enough for trigger pin)
  • Tray for screws, pins and more
  • Red loctite


Step by step

*Note: this is a universal guide, most likely you will need to refer to your rifle's manual/diagram during this process*

  1. Ensure rifle battery is disconnected and chamber is clear
  2. Unscrew/punch out front pin to disconnect upper receiver from lower receiver (Krytac uses a 2.5millimeter hex screw)
  3. Remove stock to gain access to the buffer tube screw -> unscrew it with the Long Phillips to remove the buffer tube
  4. Unscrew (2) pistol grip baseplate screws (Screw size may vary, use best fit screw head to avoid stripping)
  5. Disconnect motor tabs carefully -> remove motor
  6. Unscrew rear pin screw if needed
  7. Unscrew pistol grip screws - take off pistol grip (helps to have magnetized bit, you can try magnetizing bit by running it across the motor)
  8. Unscrew/remove magazine catch
  9. Punch trigger/center pin from right side of the receiver (shooter's perspective, check first to punch opposite of the side that has the pin's spike side)
  10. Unscrew ambidextrous selector switch from the right side of the receiver
  11. Punch rear body pin
  12. Put fire selector in the "semi" position and remove the gearbox
  13. Replace the ambi selector side gears from the stock gearbox to the UA gearbox
  14. Install gearbox into lower (note wire placement and selector positions match)
  15. Punch in rear body pin
  16. Screw in ambi selector switch
  17. Punch center pin back in (smooth side first) 
  18. Reinstall mag catch
  19. Screw on pistol grip (Careful to route motor wires in correctly and fully)
  20. Reinstall motor and place motor tabs back on (careful with orientation of positive and negative, and ensure tabs are fully slotted on and not to break off motor tabs. You will have to carefully bend the motor connectors, make sure you do not cause kinks)
  21. Check to ensure motor freely bounces in the grip and wires are in clearance to avoid stripping
  22. Install motor baseplate and screw into position
  23. Install stock/buffer tube onto the receiver (make sure you have your sling/rear plate slotted on first and that your batter wires are properly routed through)
  24. Screw on buffer tube (careful not to pinch any wires towards the base of the buffer tube as it is easy to over look. Use a flashlight)
  25. Slide upper receiver back onto lower receiver
  26. Punch/screw in front pin
  27. Ensure gearbox alignment is correct. Look down barrel and confirm the Air nozzle is sitting centered. If not centered, make sure screws/pins have been installed correctly and if still not, you may need to shim gearbox with large shims


Try out rifle and make any motor height adjustments where needed. It helps to test this on full auto, there should be no screeching. If you still experience a screech, you may need to use motor/motor grip we list as compatible or reshim gearbox if needed. 

Use spring guide spacers to increase FPS if needed.