How to Shim an AEG Gearbox Without Creating Gear Bind

Shimming sets the side-to-side position of each gear inside an AEG gearbox. The aim is controlled axle play and clean gear engagement without forcing the gearbox shell closed. A gearbox that spins freely when open can still bind after the screws are tightened, so every check should be made with the shell assembled in the same way it will be used.

Before you start

• Clean the shell, gears and axle supports.
• Confirm that every bushing or bearing is seated and secure.
• Keep shims sorted by thickness and inspect them for damage.
• Remove the spring, piston and tappet components for the first gear checks.

Work from the motor side

Begin with the motor pinion and bevel gear. Set the motor height so the contact pattern is sensible, then adjust the bevel gear so it does not rub the shell or move excessively along its axle. Add the spur gear next and check its mesh with the bevel gear. Fit the sector gear last, leaving enough clearance for the spur gear and for the surrounding piston and tappet components.

Close the shell for every final check

Tighten the gearbox screws in their normal positions and rotate the gears by hand. They should turn without a hard point. Check each axle individually: excessive movement calls for another shim, while resistance after tightening usually means the stack is too tight or a support is not seated. Do not judge the setup by sound alone.

Reassemble in stages

Add the piston and tappet plate only after the gear train moves freely. Manually cycle the system and verify sector-to-piston engagement, tappet movement and clearance around the shell. Powered testing should be brief and should stop immediately if the system binds or the engagement pattern is uncertain.

Shimming is part of a complete build, not a shortcut for mismatched gears, damaged bushings or incorrect motor alignment. When choosing replacement parts, compare JVAN airsoft gear sets by platform and configuration.