Retro Arms aluminium CNC gearboxes

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Let's talk about competition between aluminium CNC gearboxes.

1. Retro Arms ver.2 QSC gearbox with included full-metal spring guide

Just about 7-8 years ago, they were quite rare. One of the first pioneers was the modular gearbox made by Pro-Win which appeared to be like something from NASA back then (because of the price as well). The time went by and after a few more years there were more and more new CNC gearboxes from Chinese brands such as SHS, X High-tech, B&C and others. Which are to be exact, weren't completely machined from the single aluminium block. Moreover, many of those brands came from the same actual factories. Surely, there were also fully machined examples from small custom workshops or local artisans who found their ways to the CNC milling machines.

Today CNC machined gearboxes became a common parts and the airsoft market is full of different offers. The competition between the manufacturers around the world had a positive effect on a gearbox sale price, which went down to $110-130 at the average. And now, as we'll see below, competing companies are trying to excel each other with the quality of their products.

2. Retro Arms ver.2 QSC gearbox with included full-metal spring guide

Let's review this on two CNC aluminium gearboxes: Army Force and Retro Arms. Sure, they are equal in their purpose and strength for the extreme mechanical stress. At least I never saw a broken CNC gearbox but if you have - please let me know. At first glance, the difference between them is in precision and small details. However, sometimes, the small details make BIG differences.

Cylinder side play
In the Army Force / Combat Gear / X High-tech gearboxes the cylinder can have a sever side play at the back. This can result in compression deprivation and accelerated wear of cylinder inner surface due to the contact with the piston's teeth, carrier slides and head. The consequences will be grieve if the back of the cylinder won't be centred properly and side play fixed.

3. Cylinder side play inside the Army Force gearbox

Combat Gear / X High-tech gearboxes may have another problem: the frame angles won't let you install the cylinder. In this case, you have to file the frames manually.

4. Frame parts of the Army Force gearbox that might interfere with the cylinder

Retro Arms gearboxes have special notches in these zones (marked blue). The cylinder lies precisely in its housing. The cylinder head also fits perfectly in Retro Arms case, while it might require modifications on the sides to fit in the Army Force gearboxes (similar to the problem which occurs with the regular die-cast Classic Army gearboxes).

5. Retro Arms ver.2 QSC gearbox with marked notches

6. Precise emplacement of the cylinder inside the Retro Arms ver.2 QSC gearbox

Width and shape of the guiding rail
Certain Combat Gear / X High-tech gearboxes have piston guide rails machined too thin and rounded. This also leads to noticeable side play of the piston inside the gearbox with all ensuing consequences.

7. Guide rails in the X High-tech gearbox

As you can see, there is no such problem in Retro Arms gearboxes.

8. Guide rails in the Retro Arms ver.2 QSC gearbox

Now I need to emphasize that I'm describing the gearboxes, which I got my hands on. When I searched the other X High-tech photos on the web, I haven't spotted this defect. However, the risk is there and it would be really pity if it's you who receive the gearbox with this kind of flaw, especially considering the price.
Retro Arms excludes any possibility of such a situation, providing lifetime warranty for all their gearboxes. Which means the manufacturer has a complete confidence in the quality of his products.

Bearings
Chinese gearboxes come with the sets of ball bearings (usually 8mm or 9mm).

9. 8mm bearings (6 balls) of the Army Force gearbox

You have to buy them separately for Retro Arms cases.
They fit in very well, tight enough for the outer rings not to rotate inside their slots.

10,11. 8mm bearings (7 balls) of the Retro Arms ver.2,5 SR-25 and ver.2 QSC gearboxes

Cutoff lever
I decided to buy Retro Arms aluminium cutoff lever just for the sake of better compatibility. It fits with almost no friction and is held by the M2.5 slot screw. At first, this screw can look too big, but when in place, it's mere 0.1-0.15mm above the lever.

12,13. Retro Arms cutoff lever and serration of the switch spring pin inside the Retro Arms ver.2 QSC gearbox

On the bottom side (where the sector gear engages the lever), it's milled so it won't contact with the bearings. But on the opposite (right) side which lays under the trigger switch they haven't made the groove. That's why the switch wouldn't fit until its plastic catching part is filed a bit.
The Retro Arms gearbox itself is compatible with cutoff levers from different manufacturers.

Switch assembly
It is possible that your old switch assembly will require some minor modification in order to be installed in Retro Arms gearbox. Such as filing the angles or etc. Though it is quite often required anyway when you're combining switch assemblies and gearboxes from different manufacturers.
I also had to file the back of an old A&K SR-25 trigger in order to make its pull longer.

Screws
Most of the Chinese CNC gearboxes come with a regular Philips or Hex screws. In some cases, you need to cut the threads by yourself. Retro Arms supplied theirs with rigid M3 Hex screws and wrought threads.

14. Retro Arms Hex screws

Windows
Retro Arms gearboxes have windows... lots of them.

• three openings for lubricating the gears without disassembling the gearbox (absent in newer versions)
• windows for checking the angle of engagement between the sector gear and piston teeth
• window for anti-reversal latch release
• windows for checking the piston position
• windows for cables
• windows for bigger air flow (?)

15. Bottom half of the Retro Arms ver.2,5 QSC SR-25 gearbox

In my opinion, there's no real need in most of these openings, because they present more ways for the dust and dirt to get inside the gearbox. See for yourself:
- openings for lubricating the gears are more for lazy users. People usually buy CNC cases for heavy loads; hence, I can't imagine anyone who would only renew the gear grease without a proper maintenance and inspection of other parts in such a powerful system.
- the window for AOE check is quite useful during the tuning process, but just one on the upper side would be more than enough.
- the anti-reversal latch release window isn't that necessary especially when there's quick spring change feature which lets you dismantle it before the disassembly.
- same thing with the openings for piston position check, they're useful only during the tuning process.

The cable windows, on the opposite, are useful without any doubt.

Other features of the Retro Arms gearboxes
One of my favourite features of these gearboxes is the milled shoulder that covers the cables from the motor axle and pinion gear. In addition, underneath it there's a window, which allows you to use cables with bigger gauge.

16,17. Cable window and its shoulder inside the Retro Arms gearbox

The precise slots for anti-reversal latch and trigger holding them firmly without a chance to jump out.

The holes for central pin made a bit wider. Retro Arms claims this for better compatibility with receivers from different companies. Can't say whether this was necessary, because non-standard bodies aren't that common. Besides, even if this problem occurs, it's quite easy to fix.

There are handy small grooves on the pins (see photos 12 and 13) which securely hold the tappet and switch springs. In the SR-25 gearbox those are separate parts which are screwed in the bottom half, so it would be a good idea to secure them with a thread locker.

18,19. The switch spring catch of the Retro Arms ver.2,5 QSC SR-25 gearbox

Another kind of inserted pins with cone heads make the gearbox halves assembly easier and more accurate.

20. One of the centering pins of the Retro Arms gearbox

Compatibility
Retro Arms states that their gearboxes have some compatibility difficulties with Classic Army AEG's. I happened to have the AR-10 from CA and found out why. During the gearbox installation inside the lower receiver, the back pinholes of the gearbox and receiver are not coaxial. They are displaced by 2mm approx. Which is not that critical and can be fixed if desired.

In conclusion, let's get back to the competition questions. The price of the Czech gearboxes, surely, a bit higher than for the Chinese ones. Mostly because you have to buy the bearings separately. The Chinese CNC ver.2 gearbox with the QSC feature price tag is about $117. The Retro Arms gearbox with the bearings set will cost you $137. But this small price difference is absolutely insignificant in compare with all the quality and design superiority of the Czech gearboxes, lifetime warranty and convenience in maintaining and operation.