Retro Arms CZ Billet CNC 8mm Ver.2 Gearbox Shell for M4 Series Airsoft AEG Rifles (Model: Split Gearbox / Hop-Up Chamber)

"Wonderful gearbox. Used in high-stress build with a DSG and high rating spring. Tens of thousands of shots later, and after hundreds of full auto bursts, opening the gearbox and removing all of the old grease reveals a gearbox as good as new. Incredibly lightweight, yet robust and extremely strong. I ordered black but got sent silver, but can't complain about the quality of life that the gearbox provides with the quick change spring system, for when I need to switch out the spring for my DSG, which I need more of.

"Great gearbox. Was everything I thought it would be.

One small problem though: my buffer tube screw was too long for the QC spring guide that came with this gearbox. It was originally M5 x 50mm, had to grind it down to M5 x 40mm for it to fit correctly. Also if you're gonna remove material to fit bushing/bearings, use a hand tool and manually take off a LITTLE bit at a time. Don't go crazy with it!

"These gearboxes are absolutely beautiful and machined very well. FLT bushings fit flush on both sides although they were extremely hard to fit in the shell, I removed some material from the inside of the bushing holes to accommodate. Everything else fit absolutely fine, almost every part in my gearbox is SHS except the cylinder is LCT and the cylinder head is King Arms. This gearbox is extremely light, very strong and easy to use, with extra features making it easier to tech on. Installed a JeffTron Leviathan as a drop-in fit, aftermarket MOSFETs require no modifications to install. Overall great product! I’ll have a video review up on my YouTube channel Octane Airsoft shortly!

"These Gearboxes from Retro Arms have been a game changer in the teching world. I absolutely love how well these are machined and fit. The fit and finish alone is beautiful and is almost worthy enough just hang on a wall. The level of reinforcement that is in these gearboxes is amazing! I have been running these gearboxes in many different builds and I must say, this is top notch quality

"Picked mine up at evike on Saturday. The quality is phenomenal, I can’t describe how perfect the dimensions are. Very light compared to stock gearbox. I am using it in my dmr build and hopefully it withstands the m160 it claims to be rated at. Before this my gun shot about 550 fps and the stock gearbox eventually cracked(I knew it would break but at the time this was out of stock). The only problem I have is it fitting the stock Krytac pistol grip. Not a big issue, I was going to replace it anyway. 11 out of 10

"This is the best gearbox on the market. If you need a no B.S. shell and are willing to spend the money, you will not be disappointed. There are just too many great features on this gearbox to list. I have an entire table covered with just internals, and every single one fits perfectly into this and works flawlessly. If you're not willing to spend the money, Lonex makes nice shells for about 50 bucks.
Pros:
-Extremely light
-Grooves on tappet return spring notch
-Precise CNC cuts
-Huge range of compatibility
-There's a little shelf for the wires behind where the pinion would go in, makes installation a breeze
-When using Modify ceramics, they lay completely flat to the inside wall

Cons:
-I installed Modify ceramics into this shell, but with some holes they would fit but others they would not. I measured the diameter of everything with a digital caliper and the bearings measure ~7.98mm on average, and the shell's slots ~7.96mm on average. Its not really a con since it's still very close, but it kills me to have to put super glue on such a beautiful thing.

Overall, don't let that last part turn you away from this. It's still a fantastic shell. P.S. If you like rainbow colors, they have all different colors of these on the Retro ARMS website.

"dropped titan in there with no problem, everything fit in perfect. only issue is standard buffer tube screw does NOT fit. will need a shorter one

"This gearbox shell is by far the best full sized Version 2 gearbox I have ever worked with. I didn't have to hold anything down when closing it. The anti reversal latch doesn't shift around like some other gearboxes during reassembly. I also dropped a Gate TITAN in here without any modifications whatsoever. Every dimension of this gearbox is perfect.

Pros:
-Perfect dimensions
-Quick change spring feature
-No modifications needed
-Spring posts are narrow, with indentations to hold the spring in place. You can use thick pliers to install the tappet plate spring.
-Anti reversal latch is secured just enough that you don't have to fiddle with it when closing the other half of the shell.

Cons:
None. Really. None.

"Absolutely flawless. That's the only way to describe the quality of this shell. It's expensive as hell, yes, but there is no role that this cannot handle. I am personally running a DSG build in this for now, and I plan to get another one for my next DMR build. Just to measure compatibility, I tested the fit of numerous different manufactures' parts that I had lying around in the gearbox: G&P, Echo1, Lonex, ASG, Systema, Siegetek, BAAL and Matrix. No compatibility issues whatsoever.

PROS:
CNC aluminum (flawless and smooth inside and out)
Lightweight
Parts compatibility

CONS:
The ONLY con to this shell is that for whatever reason the screw holes for the cut-off lever and trigger contact assembly is slightly larger than average. I got lucky and while digging through my parts bin I found the gearboxes of some OLD 'Aftermath' AEG's I'd purchased back in 2008 and the screws that held the contacts into the trigger assembly ended up fitting perfectly. You may end up having to find other means to get properly fitting screws.

Overall absolutely great shell. If you can manage the price, it is totally worth it.

"First off let me say that this is THE BEST GEARBOX OUT THERE.

You will never find ANY gearbox better than this one, and you know why?

It will never break. EVER.
It is compatible with every internal part possible, and I do mean EVERY one.
It looks beautiful
For what you pay, this will do the job 100% of the time.

"It is a good gearbox, but I wish they included a tool and a manual for the parts. I do not know which piece to use for the tappet plate spring. I am disappointed in that sense

"One of the most beautifully made gearboxes on the market: precise CNC work, extremely lightweight, yet still incredibly strong due to the high quality extruded aluminum blank they're milled from. I own 3 of them...but I must say that I've had mixed results. One of my builds has been trouble free, a second one disturbingly loud no matter what parts or shimming I try, and now a 3rd one simply won't work (low compression) no matter what I do. I've done some research, taken it to a machinist friend, and discovered that the thickness of the front of the gearbox is about 1.2mm thicker than proper TM spec, and thus the nozzle can't reach the bucking for proper air seal. If you try a longer nozzle you end up with feeding problems, so the only solution is to grind off the appropriate amount from the front of the shell. This is not something you should need to do with a gearbox shell that costs over $100, and beyond the capability of most users. Sadly, this is a well documented issue and unfortunate because these are otherwise wonderful gearboxes. My go-to in the past is the reliable and accurate G&P 8mm gearbox (NOT the i5), which is now frustratingly unavailable.

"Horrible product and horrible support from the manufacturer. This gearbox failed after 2 weeks of use. Avoid it at all costs!

"just got this on jan 30, 2020 and one of the little numb that holds the cylinder head broke after around 130rds. as much as i paid for this I am so disappointed that i didn't even get a game in before it broke. I have the G&P Reinforced V.2 Gearbox Set with 8mm Bearing that ran the same set of internals without a problem and is $40 cheaper. the only reason I bought this was so I can have the QCS system without having to run the chance of messing up by modifying my G&P gearbox. should have just bought another G&P and modified it an saved some money

My internals:

S&T Standerd 18:1 gears
I dont know what cylinder but it is a full cylinder
Maxx cylinder head
Maxx high performance piston head
ShS full rack pistion
ShS M130 irregular pitch spring
Gate Titan Mosfit
King arms regular trigger
5 yrs old Matrix Magnum High Torque Performance Airsoft AEG Motor - Long Type


when it did shoot i was at 410-420 fps and 20rs

"Trust me... I REALLY wanted to give this a 5 star review, I wanted to say it was the best gearbox out there. But I can't. From day one I had problems. When It says, "Disclaimer: Please use discretion when installing custom parts as fit and compatibility may vary. Some modification may be required. Professional installation recommended." It should say, " doesn't work with anything." I have had problems with the cylinder, a TM cylinder fitting. The cylinder head pegs are too small which make the assembly wobble. The trigger trolley and contacts won't fit, Semi won't work the spring guide is press fit and can come undone. Some motor grips won't line up. I had the sand down the area for the motor grip to actually fit over. Then the whole shell wouldn't work with itself! literally the two haves never seemed to line up... I wish I had my money back....

It has great features like QD spring guide and large areas for wires. But I'm having problems with it that a 10 buck crappy china shell wouldn't have problems with. For 100-120 bucks I'd say it is MORE than fair so have better fitment with parts. I've tried many brands and all of them didn't work. I really wanted this to work but I can't recommend Retro Arms. Go with a G&P gearbox. I'm glad I didn't buy a m249 shell. I wonder what other problems would occur....