JLS FN2000 Gearbox & Disassembly

This walk through is a basic introduction in the disassembly of the FN2000 down to the gearbox, and a look at the internals as well as a review of recommended upgrades. I recommend that you read through this thoroughly before taking apart your gun, and remind you that disassembly of your AEG will void its warranty. I only recommend this to those who are either experienced with the internals of an AEG gearbox or those who have the patience to learn from their mistakes.

Tools Needed

Rubber Mallet / Hammer

Punch tool / Hex wrench

#2 Phillips Screwdriver

Flat Blade Screwdriver


Before continuing, remove the magazine from the gun, point it upside down in a safe direction and fire on semi-auto 3 times to ensure that the barrel and chamber are clear. Remove the battery. This ensures safety while working on the AEG.

upperFirst remove the upper assembly by pressing the release switch towards the left side of the gun (it is located just above the trigger). The upper will slide forward and just before hitting the end of its travel you will need to pull it up and away from the receiver. If you find that it will not come out, make sure that the inner barrel and hop up are free of the receiver and that the upper assembly is not slide all the way to the end.

inner-barrel-and-hop-upFrom here you can easily remove the inner barrel and hop up assembly by pulling the inner barrel straight out of the upper receiver. As you can tell the hop up assembly is plastic and the barrel is brass.

Now to tear it down to the gearbox. First we need to remove the foregrip. This is accomplished first by removing the pin at the front of the trigger guard. This proved to be somewhat difficult and required a bit of force. I placed a hex wrench on top of the pin and knocked it out with a few strong blows from a rubber mallet. I prefer the rubber mallet (over a hammer) as it’s less likely to damage the gun itself. With the pin removed and put in a safe place, you can look down into the lower receiver and see the four clips that hold it in place. I used a flat blade screwdriver to push them towards the center and down, releasing the foregrip.

The next step is to remove the hop up adjustment cover. This is held in place with a pin that acts as the hinge for the cover. This is removed in a similar manner to the foregip pin.

Now remove the battery cover.

dissassembly-guideWith that removed you’re ready to take out screws. The diagram below points out the pins that were removed (with red arrows) and outlines the screws that must be removed with orange circles. These screws vary in length, so I suggest taping them to a spare piece of paper in the same respective locations as they would be installed in the gun so that they do not get mixed up.

With the screws removed, you can begin to pull the two halves apart. A few things to note, it was difficult to separate the halves near the hop up chamber and it required more force and wiggling than I thought would have been necessary; two watch for the mag release button and spring and the trigger safety spring while separating the two halves. I suggest that you do this with the left side of the AEG facing up and the right side against your workbench.

internalsNow you have access to your gearbox and all the inner workings of the gun. Not much to it at this point.

trigger-mechNotice the trigger assembly. The trigger safety functions by a small nub and spring located just underneath the trigger. The trigger actually pulls a rod that is connected to the gearbox trigger. Unfortunately this rod has a tendency to flex a bit, which is what creates the spongy feeling you get with the trigger of this gun. This is inherit to the design as there is flex in the rod and movement at the trigger on the gearbox as well and unless you when to very tight tolerances this cannot be overcome easily. I found it easier to remove the screws on the cover of the trigger rod located between the trigger and the gearbox at this stage. With that removed you can free the trigger rod from the trigger and the chassis leaving it only connected to the gearbox.

left-side-of-mechboxNow take note of the wiring and the gearbox so that when you put it back together you can easily route the wires appropriately. Pull the gearbox away from the chassis. Notice that the feed tube from the mag well is not attached to the gearbox, mag well, or hop up chamber in any permanent way. Note the orientation of it so that if it slips out you can place it back in appropriately.

right-side-of-mechboxWith the gearbox removed, we can begin to take it apart. First remove the trigger rod. This takes a bit of finagling, but with careful effort it can be done without any damage to the gearbox or the rod.

Now we’re down to just the gearbox and the wiring. The wiring in this AEG is remarkably nice given the price point of the gun, but the soldering done on the wiring is horrid. If you’re handy with a soldering iron, I’d suggest you break it out, and use some desoldering braid and remove the old solder, and resolder the connections to the motor at least.

rear-of-mechboxWith the gearbox placed on it’s left side, the screws are now visible. Note that there is no access hole to the spring guide from the back of the gearbox. This will make it a bit trickier to take apart and more difficult to put back together. I place my left hand on the back half of the gearbox with my middle finger on the spring, index finger on the cylinder, and thumb on the gearbox. I press down on the spring firmly as I remove the screws starting with the ones up front near the nozzle working my way back. Note again, the screws are not all the same. Now with the screws removed, carefully remove the cover of the gearbox. While you are doing this be wary that the spring and spring guide are going to want to jump backwards on you, the sector gear might want to jump towards you, and the anti-reverse latch might just decide to click its heels and head to OZ. After lifting off the right half of the gearbox pay attention to the gears and bushings as they might have shims stuck to them and you do not want to get them out of order. Slowly lift out the spring guide and release the tension of the spring.

Ta-da! The gearbox is open and ready to be upgraded / repaired / smashed with a hammer in a release of rage.

Let’s take a closer look at the gearbox components.


gearsThey seem pretty well made (nothing amazing or horrid about them). They are stamped with the letters “DL”. If anyone has seen this in another gearbox, or knows the origin or meaning of “DL” please let me know so I can add it to this.

Tappet Plate

This seems a little too flexible to me. The moment I opened the gearbox the sector gear wanted to fly up. In the process it bent the tappet plate upward into a curve much sharper than any other tappet plate I’ve seen bend. There was no permanent damage, but I don’t know if that is a testament to its durability or weakness (doesn’t break or too weak to stay rigid). Personally I’d leave that stock till it broke.

Spring Guide

spring-guideThis was the first time I’d seen a clear spring guide in person. I think the attempt here was to make it similar to the Angel spring guide, but you can tell it’s not quite the same quality. I’d suggest putting in a better spring guide.

Piston / Cylinder / Cylinder Head / Air Seal Nozzle


Compression was excellent with no notable leaks anywhere. The piston had a sizeable and very pliable o-ring that was nice and smooth. The porting of the piston head was a welcome sight. The cylinder head was decent with nothing to complain about. The air seal nozzle did seem to be lacking in quality. There were burrs on the edges of the plastic where it was formed, and it did not fit as well into the tappet plate as I would have liked for it to. This too is a part I would suggest replacing.


The switch itself seemed to be decent quality and the soldering on the switch is much better than what I saw on the motor.


This gearbox is a modified version 2 so the motor is enclosed inside the gearbox itself. The motor is a short style motor and the shaft of the motor appears to have a “D” shape where it meets the pinion gear. The pinion gear has a set screw in the side of the base to attach it to the shaft of the motor. The performance of the motor and the outward appearances lead me to believe that for all but the most gear headed tweakers, it should be sufficient.

Gearbox Shell

The shell itself it what I’d expect in this price range. The metal is similar in appearance to what is found in other AEGs in the is price range, and are not reinforced like the Classic Army gearboxes are. I would be concerned about upgrading past a M120 spring, that the gearbox might crack too easily.


This was a pleasant surprise. All the bushings are metal and appear to be high quality. They each snapped into the gearbox shell well and fit the axles of the gears without too much play or too much friction. Definitely a keeper unless you’re looking to up your ROF and want bearings.


The gearbox had the usual dual blend it appeared. One lube for the cylinder set and one for the gear set. Either way there was a real smattering of lube everywhere. I’d highly recommend that upon upgrading, that the entire gearbox be stripped, washed out with liquid dish detergent, and relubed using Radio Shack Lube Gel for the cylinder set and White Lithium Grease for the gears.

Upgrade Recommendations


Reinforced spring guide
Air seal nozzle

Increased FPS

Reinforced bearing spring guide
Air seal nozzle
Systema M120 spring (or equivalent)
Relube the gearbox properly


This gearbox is of above average quality for a gearbox in this price range. The component for the most part, exhibit good quality control, and function well. For reliability sake, there are a few pieces that I would change and are inexpensive but one could wait till they broke to replace with no long term or extensive damage to the gun. This gearbox also lends itself decently to upgrading for increased FPS. The only concern is that anything over a M120 spring might put enough stress to crack the gearbox quickly and a replacement gearbox might be hard to come by given the proprietary design. The tolerances of the gearbox are very good making for a nice snap as everything comes together without a lot of play and fiddling about.

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10 responses

9 08 2007

I’m wondering. Do you think it would be possible to put a P90 version 6 gearbox into this body and mod the safety switch to act like the P90’s? I’ve been wondering this since this was released.

9 08 2007

Actually, I’ve never worked on the internals of a P90. I know this gearbox is similar, and the trigger is very similar. I’m not sure about moding the safety switch though. You might want to check on http://www.arniesairsoft.com/forums to find out if anyone else has done that.

8 09 2007

I have read on other websites that the front of the outer barrel can be taken off for a shortend CQB version. I believe you can see pictures of this at:


My question is do you have any advice on how to unscrew the barrel without breaking anything? Also is it standard clockwise thread , or counter clockwise? Thank you for the help.

10 09 2007

Actually, I don’t recall how it’s threaded. I do know that the orange tip was very securely glued on. Evidently you can change the outer barrel, but I’m not sure what outer barrel you would use to replace it with, or if you’d just screw the flash hider into the body after taking off the outer barrel.

11 09 2007

What size Air seal nozzle would you recommend?
I know its a mod version 2 gearbox, but I am wondering what Air seal nozzle did you use?

11 09 2007

I didn’t change mine, but if I recall correctly it’s the same as an M4.

11 03 2008

I was wondering, how hard do you think it would be to change the spring guide, and possibly the air seal nozzle? Would a version 2 M4 spring guide/air seal nozzle work for this, since it’s a modified version 2 gearbox?

12 03 2008

A version 2 spring guide will fit. I do believe that an M4 style air seal nozzle is the right one for this aeg.

19 08 2008

Hi all good review on the FN2000
i have one and read your review so i decided to put a new air nozzle and spring guide and a m100 spring but to my horror when i got the weapon opened i found that the meckbox was broken and so was the tappet plate 😦
now trying to find a new gearbox well you may as well be looking for hens teeth i have tried every were with no luck even have guys looking out for one in China – HK
so if you here off one going keep me in mind or if you know how to fix a box PLEASE HELP……

25 07 2011
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