Here's a review of the ZCI v2 complete M4 gearbox. I got it from http://www.ak2m4.co.uk, it is currently available for £55.
The gearbox comes packaged in a nice, form-fitting blue foam box. Very nice quality packaging. It's a v2 gearbox with a few special features- QD spring guide, microswitch trigger, special selector plate. I'll explain more later in the review.
There. Easy spring changing. The spring guide itself is plastic with a steel base, and comes with a washer (which I somehow managed to lose, it should pop up in my sock or something in a few days).
Looking more closely at the selector plate, you'll find that it's quite different from a traditional one.
There's no safety lever, it's all integrated into the plate. This makes it proprietary to this gearbox, but I think minimizing spring-loaded moving parts in a gearbox design is a good idea (and you'd never need to replace those parts anyway, the only time you'd buy another set is if you've lost them).
Looking inside, you can see the microswitch itself. It makes a clicking sound when pressed. The entire gearbox is well-lubricated. Some space that would normally be empty in a typical gearbox has been filled with metal. As the area experiences little stress, I wouldn't really consider it reinforcement (or an effective one, at least). It doesn't seem to be doing any harm though. The trigger is actually attached to the other half of the gearbox:
Because of this setup, the trigger basically can't pop out when you're closing the gearbox shell. This coupled with the QD spring release makes opening and closing the gearbox very easy. The gears appear to be sintered steel and the sector gear has a brass delayer chip installed. This is a first impression review, so I can't make any claims about durability yet. They were pretty nicely shimmed stock.
The cylinder head is plastic with a brass tube. Compression is neither amazing nor terrible.
The rubber pad attached to it is thicker than usual, but not enough to correct angle of engagement very much. It still adds a bit more shock absorption compared to other stock rubber pads.
The piston is clear polycarbonate with a single metal tooth, and the piston head is ZCI's standard ported nylon one. No teeth removed for angle of engagement stock. Compression is pretty good, although oddly not quite as good as the ones I've bought separately (minor variations in o-ring tolerances). The piston head is quite durable and another one that I have has held up for thousands of rounds with no signs of wear.
The tappet plate is well, functional. Not too stiff, not too flexible.
The cylinder would appear to be stainless steel.
The nozzle does not have an o-ring and is made of plastic.
The tappet plate spring seems shorter, wider, and stiffer than others I've seen. This is an improvement over most other stock tappet plate springs as it improves feeding at very high rates of fire (shortening the tappet spring is commonly done to improve feeding).
I'd call the internal parts quality okay. Not amazing, not terrible. There's nothing I'd say really needs replacing, however the gearbox shell is not radiused stock and the piston has no teeth removed. The big problem I've found is that the front hole of the gearbox (where the nozzle comes out of) needs filing down slightly in order to fit hop units (tested with lonex and G&G m4 units). Because of this, the gearbox isn't a drop-in. Similarly, I couldn't use aftermarket spring guides without either shaving down the spring guide slightly (I was able to use a ball bearing spring guide like this, the material removed isn't enough to interfere with the bearings) or widening the QD spring guide hole. It's a bit more work, not the end of the world, but not something that should have to be done.
So what's good about this gearbox? There's the QD spring guide, pretty convenient. The whole trigger system is pretty interesting and IMO the big selling point. Due to the microswitch, the trigger pull on this gearbox is roughly the same as the amount of play my traditional v2s and v3s have when on safe. The design also reduces the amount of moving parts and makes it practically impossible for the trigger to pop out. Everything fits in pretty securely, and the design really makes working on the gearbox a lot more convenient.
-much shorter trigger pull
-strong tappet plate spring
-simplified trigger/safety assembly
-QD spring guide
-everything inside the gearbox fits well
-requires widening front gearbox opening
-sintered steel gears
-no o-ring nozzle
-no aoe correction (not even second tooth removed on piston)
-no bearing spring guide
Verdict: It's not perfect, but that'd be asking a bit much given the price. There isn't anything irredeemably bad about it and all the problems are fixable (replacing suboptimal parts is especially easy given the QD spring release). Especially recommended (the gearbox shell, at least) for M4 DMR builds due to the shortened trigger pull.