Gearbox & Internal Parts

What is in a gearbox

An AEG gearbox turns the motor's rotation into a piston stroke: the motor drives a bevel, spur and sector gear, the sector gear pulls the piston back against the spring, and the piston drives air through the cylinder and nozzle into the BB. Everything in this category is one of those parts or supports them, such as bushings or bearings for the gear axles, the tappet plate that moves the nozzle, the spring guide and the shell itself. Version 2 gearboxes sit in M4, MP5, G3 and SCAR replicas; version 3 in AK, G36 and AUG; V6 and V7 are P90 and M14 designs. Use the gearbox version filter first.

Common upgrades and repairs

A stripped piston or a cracked shell is the usual reason to open a gearbox. While you are inside, steel gears with a proper shim job, a polycarbonate piston with a metal rack, an aluminium piston head with bearing, a brass cylinder matched to barrel length and a bearing spring guide turn a stock gun into a reliable one. A stronger spring raises energy: M90 to M100 is standard, M110 to M120 for 1.3 J, M130 and up for DMRs where the site allows. Cylinder and barrel should match in volume for efficiency.

Before you start

A gearbox build needs tools from maintenance, a motor that suits the gear ratio and a MOSFET if you plan to run 11.1 V. Internals are unrestricted parts, and the energy of the finished gun must stay within your country's limit.