Package contains the split gearbox, all necessary screws, ULTRA Hop-Up Chamber, and a spring guide.
The ultimate solution for players who want a fully customizable Hop-Up chamber.
The Hop-Up Chamber ULTRA is based on our BASIC chamber and is also manufactured on precise CNC machines from aviation-grade aluminum 7075. Like the PRO chamber, it includes an adjustable feeding shaft.
New features include an adjustable nozzle shaft, designed to shift the chamber slightly forward when necessary. This is particularly useful for tuning the chamber’s longitudinal position—helping to improve feeding reliability or alignment with the magazine. The position can be adjusted in 0.2 mm increments using the included shims. Please note that this feature may require the use of nozzles with different lengths. The adjustable nozzle shaft will be available in multiple outer diameters, allowing you to choose the best option for your gearbox shell.
Another new feature of the ULTRA chamber is the inner barrel securing mechanism. Compared to standard designs, we completely redesigned this system to address a common issue: the C-clip’s inability to hold the barrel firmly in place, causing unwanted movement or rotation.
Our solution includes a reshaped C-clip made of POM and a custom union nut. This nut secures the C-clip tightly to the barrel and is precisely machined to reliably center the inner barrel. With this design, the C-clip cannot loosen due to barrel rotation, and the barrel cannot move longitudinally because it is firmly held by the union nut. Tighten the nut by hand only! The union nut has a maximum outer diameter of 12.95 mm, which should fit most outer barrels. However, please verify compatibility before purchasing.
Another upgrade in the ULTRA chamber is the shape of the main shaft. Compared to the BASIC and PRO chambers, the bore around the Hop-Up rubber seat has been flattened. Standard AEG rubbers are meant to be used here with the included rings. These rings compensate for the removed material and can be adjusted—for example, sanded—to suit different Hop-Up rubber dimensions. This modification also allows compatibility with our new ROBUST Hop-Up rubber.
CNC split gearbox V2 designed for airsoft replicas AR15 (M4), M16, and others with a V2 gearbox. This gearbox is manufactured from one solid block of aircraft aluminum (Dural) EN AW 7075 T651, which has higher strength (590 MPa) than common construction steel (390 MPa). Because of the material used, our gearboxes have better durability and resilience than common zinc-aluminum gearboxes. We manufacture these gearboxes on our precise CNC milling machines in our factory. The detail in brackets (8mm) refers to the diameter for bushings/bearings.
A split gearbox means that this gearbox is designed as two parts: Upper and Lower. Having a split gearbox has many advantages, such as easier installation of the inner parts. When setting up your cylinder set, there is no need to remove the whole gearbox from your receiver—only the upper part needs to be dismounted, as it is mounted on one receiver pin.
Recommended nozzle length is 21.3mm. Be aware that nozzle length can be influenced by many factors, for example, the angle of the tappet plate or the different length of the front part of the Hop-Up bucking. In these scenarios, a different nozzle length may be needed.
Package includes: CNC Split Gearbox V2 (8mm) – QSC in the chosen color, set of screws, and QSC spring guide.
We have made some useful improvements to our CNC split gearboxes V2:
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Spring holders have grooves, so the springs for the tappet plate and trigger switch will not jump off.
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Added groove for the safety lever, so it stays in place during installation.
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Added groove for the trigger spring, for better fitment.
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AOE control hole.
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Sockets for bushings/bearings have a smaller diameter of about -0.02mm (7.98mm), to prevent the bushings from overturning during installation. The socket edges are chamfered for easier installation.
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We enhanced the grooves for wiring by 1mm; you can now use wiring with a 2.5mm diameter.
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Added a protective edge for wiring near the motor.
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All threads are now formed instead of cut for better strength.
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Both gearbox parts are centered by milled grooves and edges.
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Added a hole for releasing the anti-reverse latch.
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For better durability, we added more material around the cylinder head area of the gearbox. This was the most critical part, and we added 100% more material where there is a large radius.
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Added groove for the anti-reversal latch spring, so it will not jump off.
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Outer edges are now chamfered.
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You can find the type of sockets the gearbox has on the outside (8mm or 9mm).
RetroArms gearbox has a quick spring change system (QSC), where you can remove the whole spring guide with a hex key and change the spring without opening the gearbox. In this design, you need to secure the spring guide with a screw—commonly, in AR15 (M4) replicas, this is done with the stock screw. In situations where you have folding stocks or no stock at all, you must secure the spring guide with a QSC screw. Otherwise, the spring guide may get stuck with the spring. This is because the spring guide is not fixed in the gearbox but is designed for easy removal with a hex key.
The space for the trigger switch is designed for compatibility with trigger switches and trigger units from common manufacturers, for example, Gate, etc.
CNC gearbox V2 has protective anodizing, even though it is difficult to anodize aluminum EN AW 7075 T651 (Dural) due to its high strength and hardness. Without anodizing, this material has a high risk of corrosion (large gray spots with a rotten appearance).
The CNC gearbox can only be installed in airsoft replicas with the Tokyo Marui-compatible standard. This applies to inner parts as well. Do not use excessive force during installation. Use only a 1 Nm tightening torque on screws; otherwise, the screw heads may strip. There is no need to overtighten screws—if you want to ensure the screws stay in place, use thread locker or thread sealant.
You can use any spring of your choice in our CNC gearbox, but be aware that stronger springs will result in reduced durability.
!! Do not use MAXX cylinder head !!
Installation of the inner parts and gearbox requires professional knowledge, experience, and skills. There are many airsoft manufacturers on the market, and each creates parts with their own dimensions—there are no universal engineering standards like ISO in airsoft. This is the main issue with many inner and outer parts from different manufacturers; they may appear to be the same product but have different dimensions. We recommend using only parts from manufacturers that follow the Tokyo Marui-compatible standard to ensure compatibility with our CNC gearbox.
