Thought I would share my thoughts on a gun I recently had the opportunity to work on. It was a repair as opposed to a build/upgrade, so all internal parts pictured are stock.
The repair, sample size 1 so far, was that the back of the gearbox shell broke where the quick change spring guide rests against, ultimately causing the spring guide to lean, and lock the piston and spring all in one.
But on to the internals:
As mentioned above, it is a quick change spring system, which is accessible without having to remove the gearbox. Just remove the buffer tube screw and washer, remove the buffer tube, and you have access to the 90degree quick change spring guide. This is akin to many of King Arms newer offerings.
The quick change spring guide has ball bearings, looks to be made of pretty solid materials and thus has little/no need for any upgrade in that department later on.
It has clear insulation wiring, with a small basic mosfet fairly close to the back of the gearbox.
The motor was a King Arms HQA series, featuring Neodymium magnets, a helical motor arm, D type pinion, vented can, and a CNCed aluminum endbell.
Then finally inside the gearbox, it has:
-SHS-esk 18:1 gears, with a tappet delay chip, shimming being somewhat loose
-8mm bearings
-full metal tooth rack piston
-polymer ported piston head(can't say if POM or other)
-brass 3/4 ported cylinder
-red King Arms tappet
The 8mm QC gearbox itself has the old styling of "reinforcement" of added material around the gears, which would render lower ratios to require heavy modification for installation. (Aside from Seigetek low ratios)
The rest of the parts aren't much different from other standard AEGs.
The fix I landed on for this specific guns misfortune was to just drop a small chunk of metal as a shim so the QC spring guide had something the be tightened up against when installing the buffer tube. Not ideal, but a lot quicker and cheaper than having to transfer all of the parts to a new gearbox shell.
