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[Tutorials]
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M4 Take apart guide
- 9/29/07 - by 
What this tutorial covers
This tutorial covers how to disassemble a standard M4 with a version 2 gearbox and explains how to replace the motor, spring, piston and spring guide. The M4 used in this tutorial was a TeamSD brand AIM M4 [you can find a review for this weapon here]. The tutorial itself is simply a take apart guide for the M4, you should be able to replace parts while the gun is down. Make sure to read this entire tutorial before you begin. Gearboxes can be a very tricky subject and you should have a working knowledge of them and how they go together before taking one apart. Lets get started.
The AIM M4 - image from evike.com
Removing the Retractable Stock
Located on the bottom of the retractable stock is a spring loaded pin (arrow). Pull it out as far as you can and slide the rear part of the stock back and off of the forward part. If the rear is not letting go of the front you are not pulling hard enough on the pin. You may need pliers.
Pull Spring Loaded Pin on underside Down
Once the rear part of the stock is removed we will now remove the front part of the stock from the weapon case itself. There is a very small allen head screw on the bottom side of the stock tube. Remove it, this will allow the end plate to be removed from the stock tube.
Pictured: End plate; allen head screw; stock tube
You will next need a long phillips screw driver to reach down into the stock tube and remove the screw (pictured below) that attaches the stock tube to the weapon (this screw actually bolts directly into the springuide that is inside the gearbox).
Pictured: Screw that attaches stock tube to weapon/gearbox
Order of disassembly for stock tube
Removing Motor and Hand Grip
The next step is to remove the motor from your weapons hand grip. On the bottom of the hand grip there are two screws that need to be removed. Do not remove the small allen head screw in the middle, you will only adjust this if you install a new motor which needs to be properly positioned.
Remove two outer screws
After the screws have been removed, remove the hand grip end plate (be careful not to loose the small metal shim in the end plate, this is crucial to the operation of your weapon and is used to adjust the motors resting position while in the weapon.
Remove end plate
Remove the lead wires to the motor. Be careful not to damage the lead posts on the motor, they can be delicate.
Remove the motor by sliding it out of the hand grip.
Slide motor out of hand grip
With the motor removed, look into the hand grip and remove the two screws that attach it to the bottom of the gearbox.
Remove two screws that fix the hand grip to the gearbox
Remove the hand grip. Make sure to pull the wires out of the handgrip with care.
Opening the Weapon Case
First remove the mag release button by using a small phillips head screwdriver. Also remove the spring that is under the button.
Now flip the casing over to the other side and remove the other side of the mag release button (left arrow). Also remove the gearbox retaining pin (right arrow); NOTE: the pin can only be pushed through from one direction. It must exit the same side that the safety selector switch is on the AIM M4, it could be different on your model, make sure before you force it the wrong direction.
The third step is to pull the main receiver pin. The main receiver pin is located just behind the trigger. Once removed it allows the upper and lower receiver to be "cracked". You now have access to the gearbox.
Removing the Gearbox
You can now remove the gearbox from the rifle. While doing so make sure that the wires do not get caught on anything as they are routed through the hand grip. The gearbox pulls straight up and out of the lower receiver.
Remove the Gearbox
Opening the Gearbox
Most gearboxes are held together by screws such as the one pictured below. Others may have some type of friction slide on the top which will have to also be removed.
Remove the screws from the gearbox
Inside the Gearbox
When you open the gearbox it is important to remember that the spring is already compressed. Always wear eye protection when in this type of environment. While keeping a finger on the cylinder (brass piece), to keep it from letting the spring decompress and fly off somewhere, use another finger to pull up on the spring guide (removing it from the gearbox).
Remove the spring guide.
After the spring guide is removed you can pull the spring and piston out as well.
From left to right (above); cylinder (brass), piston, spring, spring guide
The Piston
Now that the piston is removed from the cylinder you can unscrew the piston head from the cylinder.
Left: Old Piston with Piston Head half screwed off; Right: New Piston and Piston Head
After the piston head is removed you will notice a small metal piece (pictured below on the right) that is inside the piston which can now be removed. This is what the piston head screws to.
Piston disassembled
Close up view of the new Piston after the piston head has been removed
Yet another picture of the Piston (new and old) and piston head. NOTE: that the small metal piece is not in the proper sequence to be put back together in this image.
Putting your M4 back together
We did not make specific instructions for this section because it is basically every step before it in reverse. Just make sure that everything is set right in the gearbox before you put it back together. Also, when installing the new or old motor make sure that you hook the proper wires to the proper posts on the motor (red to positive, black to negative).
Setting the Set Screw
After getting the motor installed and everything back together you should take a test shot. If the motor is not meshing with the gears in the gearbox you will hear a high pitched grinding noise. This means you need to tighten the set screw. Do this by turning the gun over so you can see the set screw on the bottom of the handle. To tighten it rotate it clockwise. If your rate of fire is very slow then it is possible the set screw is to tight, loosen it just a little at a time. You don't want to loosen it so much that the motor gears begin to strip.
clockwise tightens, counter-clockwise un tightens the set screw.
Final Thoughts
I hope this tutorial has been useful for you. Good luck and Mod On!