How to Shim Your AEG Gearbox – Step by Step from the Bevel Gear

Do you hate loud and screechy AEGs? Maybe it’s time to shim your gearbox! 😄

Shimming your gearbox is one of the most important maintenance tasks for your replica. A well-shimmed gearbox runs smoother, quieter, and lasts longer.

For beginners, shimming can feel intimidating, but with patience and the right approach, it’s completely doable.

In this guide, we’ll walk you through step by step how to shim your gearbox, starting from the bevel gear, and what you’ll need.

Tools & Materials

• A disassembled gearbox
   
• Shims (1 set is usually enough for one replica)
   
• Screwdriver or hex set (depending on your gearbox screws)
   
• Isopropyl alcohol + cloths
   
• AEG gear grease
   

Patience 😅

 

Preparation

For disassembling your gearbox and removing the piston, cylinder, and small parts, there’s plenty of information on YouTube. This varies per brand and type of AEG.

Tip: search for

“[Your brand] AEG gearbox disassembly” – The Tube is your friend. 😉

After disassembly, clean the gears and gearbox shells thoroughly with isopropyl alcohol and a cloth, removing all grease and dirt.

Preparation

For disassembling your gearbox and removing the piston, cylinder, and small parts, there’s plenty of information on YouTube. This varies per brand and type of AEG.

Tip: search for

“[Your brand] AEG gearbox disassembly” – The Tube is your friend. 😉

After disassembly, clean the gears and gearbox shells thoroughly with isopropyl alcohol and a cloth, removing all grease and dirt.

Step 1: Bevel Gear (The Heart of Your Gearbox)

The bevel gear has direct contact with the motor, so the height alignment with the motor pinion is crucial.

How to start:

1. Begin with 1 shim under the bevel gear.
    
   • Preferably a slightly thicker shim as a starting point.
      
2. Place the bevel gear in the lower shell of the gearbox.
    
3. Slide the motor/pinion in place (see photo) and check the height.
    

What to watch for:

• The bevel gear must contact the motor pinion properly.
   
• Too low → the gear shaft tilts.
   
• Too high → poor contact, noise, and extra wear.
   

Goal: the bevel gear spins smoothly, straight, and engages correctly with the motor pinion.

 

Step 2: Spur & Sector Gears (Bottom Shim)

Once the bevel gear is aligned, we adjust the bottom of the spur and sector gears.

Objective:

• Gears must not scrape the shell
   
• Gears must engage correctly with each other
   
• Gears must not sit too high
   

How to do it:

• Place the spur and sector gears in the lower shell
   
• Shim the bottom until they sit freely without rubbing
   
• Check by rotating them by hand
   

Tip: adjust one gear at a time to avoid confusion.

 

Step 3: Top Shim (Finishing Touch)

Next, adjust the top of the gears relative to the upper shell.

Steps:

1. Place the upper shell without top shims.
    
2. Use three screws (for V2 gearboxes) to apply even pressure on the gears.
    

Adjust if you have a different gearbox model.


 

Measure clearance:

Press the gear shafts from the bottom and check the clearance at the top.

 

Add shims:

• Unscrew the shell
   
• Place top shims of the appropriate thickness
   
• Repeat until there is a tiny bit of play
   

Guideline: about 0.1 mm play. Gears must spin freely but not wobble.

Tip: start with thin shims and take your time!

 

Spin Check

Once all gears are shimmed, do a spin check:

• Rotate the gears via the cylinder opening
   
• They should spin freely and smoothly without scraping

  Step 4: Lubricate the Gears

  Apply a small amount of AEG gear grease to all gear teeth:

• Too much grease → extra friction
   
• A thin layer → less wear, quieter gearbox

• Step 5: Reassemble the Gearbox

  Reinstall all parts:

• Piston + piston head
   
• Cylinder + cylinder head
   
• Nozzle / tappet plate
   
• Trigger parts
•  
• Spring + spring guide
   

• Check while tightening screws that the gears still spin smoothly.

  Step 6: Test Your AEG

• Reinstall the gearbox in your AEG
   
• Test on semi-auto
   
• Listen for noise and check smoothness
   

• A well-shimmed gearbox is immediately noticeable:

• Quieter
   
• Less rattling
   
• Smoother and tighter operation

Done! 🎉

✅ Congratulations! Your AEG is now shimmed starting from the bevel gear, ready for a smoother, quieter shooting experience.