I purchased this Gearbox to go in an A&K Masada. The stock gearbox was causing me nothing but problems and I was never able to get it working despite putting a lot of time and effort into it.
The only change that I needed to make to the JG gearbox was to change the air nozzle to a Masada length nozzle. The internals of the gearbox are of decent quality and much easier to work with than the stock A&K box. The gearbox is already shimmed pretty darn well so unless you need to replace a nozzle or downgrade the spring, there's no reason to even open it up.
Performance-wise, I noticed an immediate difference. The sound of the gearbox operation was much smoother and quieter than the stock gearbox. When previously the old gearbox would lock up frequently, the JG gearbox was consistent and the rate of fire improved because the motor wasn't working as hard to pull the gears. The spring in this box is either an M120 or M130 (I know the description says M130 but that could just be referring to what the gearbox can handle) so the FPS is probably 400+ depending on the AEG it's used in).
All in all, this is a great gearbox that is easy to install and in most cases won't require you to open the gearbox to start using it. This gearbox may be lacking a few features of higher end gearboxes (like the 8mm Ball Bearings on the G&P gearbox), but it is almost half the price and performs just fine.
-Easy to Install
-Comes with M120 or M130 (no need to upgrade spring)
-Wiring is compatible with the stock wiring in most guns
-Fairly easy to work on internals
-6mm Bushings (no ball bearings)
-Spring may make your gun too hot (400+ FPS) depending on your field rules (not really a con, but worth noting)
Rating: [5 of 5 Stars!]