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Replace an AEG Piston

Replace an AEG Piston

Get that gearbox back in action

You’re in the middle of a game and all of a sudden, your AEG stops firing. The motor turns, the gears sound like they’re spinning, but the piston isn’t cycling. You most likely have a stripped piston and it’s one of the more common failure points in AEGs. Luckily, it’s easy and pretty inexpensive to replace your piston and get yourself and your gun back in the game!

One of the easiest ways to diagnose a stripped piston without removing the gearbox from your gun is to fire it while looking at the air nozzle. If the air nozzle moves back and forth while the motor cycles, you’re most likely looking at a stripped piston as opposed to stripped gears. If the air nozzle doesn’t move while the gun is cycling, you probably have a gear problem, which we’ll cover in a later issue. For this photo guide, we’ll be swapping out the piston in a V2 gearbox, commonly found in M4 and MP5 AEGs.

Replace an AEG Piston
Here you can see the damaged piston. The first tooth is visibly shredded on this one.

MISSION: Replace the Piston
WORK TIME: One hour
TOOLS NEEDED- Phillips Screwdriver and Grease

The first step is to remove the gearbox from the gun. Since each gun has a unique build, you’ll have to find the specific disassembly instructions for yours on the internet. Once you tear down the gun, your gearbox should look like this; ready for you to open up.

Step 1
Replace an AEG Piston
With the gearbox out, you now need to undo all of the appropriate screws holding the gearbox closed. With this gearbox, you need to undo the bolt catch system, shown here.

Step 2
Replace an AEG Piston
Unscrew the screw at the rear of the gearbox which holds the spring-guide in place.

Step 3
Replace an AEG Piston
Continue to remove the rest of the screws holding the gearbox closed.

Step 4
Replace an AEG Piston
Place one of your screwdrivers into the spring guide to prevent it from popping out of the gearbox while you open it up with another screwdriver.

Step 5
Replace an AEG Piston
Here is the gearbox opened. Don’t worry about getting in over your head, we’re here to help. Remember to keep that screwdriver in the spring guide while you take the spring out of the gun.

Step 6
Replace an AEG Piston
Remove the spring guide, spring, piston, cylinder, cylinder head, air nozzle and tappet plate from the gun.

Step 7
Replace an AEG Piston
If you plan on using the same piston head, unscrew the piston head from the piston body.

Step 8
Replace an AEG Piston
When installing the new piston head, use a little liquid thread-locker to ensure it stays on there.

Step 8 Alignment
Replace an AEG Piston
When assembling the piston unit, ensure that the various parts are installed in the correct order, as pictured above.

Step 9
Replace an AEG Piston
Be sure to use 100-percent silicone lubricant on your O-ring.

Step 10
Replace an AEG Piston
Take this opportunity to check your air seal by placing your finger over the air nozzle and pushing the piston, equipped with piston head and O-ring, quickly into the cylinder. It should have resistance when you push it in, which is a sign that you have good air seal. If it pushes right in with little resistance, you have an air leak.

Step 11
Replace an AEG Piston
Install the cylinder and piston assembly back in the gearbox, ensuring that the tappet plate is properly aligned.

Step 12
Replace an AEG Piston
A small amount of grease will help your piston slide smoothly during firing.

Step 13
Replace an AEG Piston
When you reassemble the gearbox, remember to use a screwdriver in the spring guide to keep it from popping out of the gearbox shell.

Step 14
Replace an AEG Piston
Make sure the wiring doesn’t get pinched by the gearbox shell when closing it up. You may need to use another screwdriver to ensure the gear axles line up with the bushing holes.

Step 15
Replace an AEG Piston
Take the screws out of your tray and begin placing them in their corresponding holes in the gearbox shell. Tighten everything up using the appropriate tool.

Once the gearbox is closed and tightened up, I like to test for smooth piston operation by pushing it through its full cycle with a tool like an Allen driver that I insert into the air nozzle. This will help identify any issues with the piston catching before you fully reassemble the gun.

Now you can reinstall the gearbox back into your gun and you’ll be ready to go! Ensure that the gun is properly firing with your new piston and check your FPS before heading onto the field. You should see an increase in velocity, especially if you improved your air seal over stock. Congratulations! You just added a valuable skill to your airsoft tech toolbox!

G&P, (available at

Words: Jeremy Hendricks Photos: Greg Vogel