User login

Follow us at

SRC M4 Upgrade Gearbox Set

SRC M4 Upgrade Gearbox Set Review

Airsoft is not just about externals and aesthetics, internals are the heart of our Airsoft guns and with time and use they tend to get worn and needing of replacement or repairs.

Have you ever wanted to repair/replace you whole gearbox without going through the hassle of selecting all components by hand, ordering them from different retailers, waiting to have them all together, assembling them by hand, and then discovering issues and incompatibilities between internals from different brands?

If you have then you know for sure what I’m talking about. If you haven’t but are considering gearbox upgrades, then you should read this.

SRC is a company from Taiwan who manufacture quality Airsoft internals also have great Airsoft guns. This "Gearbox Upgrade Set" like they call it is the way to a hassle free-upgrade for your gearbox since it is nothing but a drop-in one-step process, almost as simple as replacing you hop-up, with no technical knowledge required. All you have to know is how to remove your gearbox from your AEG and for that there are tons of videos on YouTube.

Plus, if you want to get adventurous and have one that can fire at over 400fps, then you can install the M120 spring included in the package which for me was really a great addition!


This gearbox is clearly reinforced near the piston head and it has a very strong and durable steel metal shell.

Also, the 8mm bearings really make a huge difference when comparing to the original 7mm bearings on my old KA gearbox in terms of gears rotation and thus in terms of performance, since clearly trigger response with this gearbox is much, much better.

The only thing it could have more is the quick spring change system.


Installation was pretty straightforward, the gearbox just slipped into the KA lower receiver like a glove.

However, when closing the upper, the story was different since the charging handle was getting stuck on the gearbox. Turns out that I had to sand a bit the part of the gearbox where the charging handle is secured, since it was way too high for the King Arms body.

After this small intervention, I was done with the installation and ready to try it out for the first time with the M120 spring installed!


All the gears are very well machined, the gearbox comes with little or no grease, so you should use some grease or better yet silicon grease in order to avoid unnecessary wear of the components.

In terms of internal goodies, this gearbox also comes with an aluminium spring guide with bearing and an aluminium piston head with double O-ring and bearing, making the travel of the piston smooth but at the same time the double O-ring manages to get a tremendous air seal and push the air through the aluminium piston head, which also has double O-rings to avoid any air leaks.

One of the things I did find odd was the fact that in the box there was mentioned an aluminium nozzle with O-ring, however the nozzle included was clearly made out of nylon fibre or some other plastic compound.

Perhaps this was just a typo on the product box and it really had no implication on the overall performance of the gearbox.

This results in great performance, even with the less powerful spring installed.

Apart from this, you’ll be getting a pretty much standard trigger assembly, and nylon tappet plate.

Another nice thing I liked immediately about this gearbox is the fact that it comes rear-wired which is the way I like to run my AEG’s, with Lipo batteries on the stock.


As you can see in the following video, with the original spring I get around 380fps and with the M120 I get 400-410fps! As for rounds per second, when using the 7.4 Lipo I get a modest 8-9 RPS, but as soon as I change it to an 11.1v thinks improve up to 14 RPS.

I did notice some wear on the piston's teeth when I moved to the M120 spring, an 11.1v Lipo and the Classic Army High Torque Motor. So I quickly changed back to the M100 spring. In case you are thinking about using only the M120 I would really recommend buying a full metal teeth piston because the M120 spring is very, very strong!

Now with the original spring and a normal motor, the trigger response is just awesome! And it’s really a pleasure to shoot with this gun. The only thing that it needs now is a precision barrel to tighten up the grouping of the BBs.


In total, with this gearbox, I achieved exactly what I wanted: a simple one-step, uncomplicated upgrade from my old King Arms gearbox.

I have won in terms of quality of the internal components when comparing to the stock internals of the KA mechbox. And most of all I now have a much more performing gearbox that has an awesome trigger response, especially with the original M100 spring installed.

For more information and local dealers contact SRC.


  • One-step easy upgrade
  • Included M120 Spring
  • Quality internals
  • Performance


  • Could have come with a full metal teeth piston for the M120 spring
  • Would have been nice to see an easy spring replacement system