Airsoft Tune-up Guide

Pictures coming soon. . . 

Upgrades and aftermarket parts

I often compare Airsoft to the car industry. Airsoft guns and cars have quite a few important similarities. For example both Airsoft guns and cars have a massive aftermarket support. Why is this important you might ask? Take the 2005 Mustang for example. The 2005 Mustang was the pinical of affordable American muscle. Like most Airsoft guns the 2005 Mustang performed great from the factory. But it was not perfect by any means. So enthusiasts sought to create improvements in the form of aftermarket parts or upgrades. With these upgrades Mustang owners were able to take their car’s performance to a whole new level. With a wide variety of parts available one could tailor his/her own car to suit any desire whether that be on the track or the street.

In Airsoft it is no different, whether you want to reach out to incredible ranges or shoot ropes of plastic there are upgrades that will vastly increase your guns performance. Whether you want to turn heads or dominate the field upgrading your gun is the best way to go.

Modifications and tuning

Like with cars increases in your guns performance doesn’t always come in the form of shiny new parts. Modifications are just as if not more important than upgrades. Without some modifications that new high torque motor you just bought will tear your stock gun apart. So what do modifications do and why are they needed? Modifications come in the form of modifying (duh) or tweaking stock parts to make them perform better and more importantly more reliable. upgrades and modifications go hand in hand. A modified gun without upgrades will see very little performance increases. And in retrospect an upgraded gun without mods will likely tear itself apart often quite violently. A well tuned gun with some upgrades will always outperform anything stock from the factory.

Modifications should always be done before you think about upgrading your rifle. Modifications while being very inexpensive or even cost free require time and knowledge. In this guide we will cover the most common and essential modifications required to install most aftermarket upgrades.

Motor height correction and gearbox noise/whine

Lets start off by going over exactly what motor height is. Motor height in your gearbox is exactly what it sounds like, its the height of the motor in the motor grip/gearbox. The height of your motor is important because it controls how well the pinion gear and the bevel gear meshes. The bevel-pinion is one of the most crucial points of interaction within your gearbox as it undergoes the most stress. This is because the transfer of energy between the bevel gear and the pinion is on two different planes.  Most Airsoft guns come with a decent motor height but if your gun whines the it is likely due to improper motor height.

The motor height is regulated by a screw located in the motor grip or motor cage (V2/V3 respectively). Tightening the screw will lift the motor and vise versa. Fining the right height requires you listen to the gearbox as you turn the screw, too high and the gun will whine too low and the gears won’t mesh properly. It is also a good idea to use a NiCad instead of a LiPo when adjusting your motor height as you will be able to hear the gearbox more accurately.

Around 90% of the noise you hear from your gearbox comes from the gears. Even with a perfect motor height improper bevel gear height caused by poor shimming will cause loud whining. To make your gearbox dead silent you will have to shim your gearbox. No stock gun comes perfectly shimmed, even guns like KWA’s that supposedly don’t need shims will greatly benefit from good shimming.


As I briefly covered in my M4 upgrade and modification guide shimming is the process of removing gaps between the gears and the gearbox shell. This is done by placing little metal washers called shims on the axles of the gears. The goal is to eliminate side-to-side movement while still allowing the gears to spin freely. Depending on how well the gears are shimmed gearbox noise can be greatly reduced if not eliminated. Adding shims onto the gears also allows you to position them up and down in relation to one another. This means you can optimize the meshing of the gears and more importantly adjust the height of the bevel gear to mesh with the motor pinion.

Shimming is one of the few modifications that requires you to buy parts. You will be required to buy aftermarket shims as most stock guns come will large poorly made shims. Aftermarket shims such as SHS shims come in a variety of sizes and are mads of higher quality materials. Aftermarket shims also come in much smaller sizes compared to their stock counterparts. This allows much more accurate shimming. SHS shims are preferred by many tech as they come in different colors and shapes to differentiate between sizes.

Shimming gears to the gearbox

To start shimming take everything out of the gearbox with the exception of the gears  and bushings (or bearings if you have them). Starting off you want to shim the gears to the gearbox. To do this take one gear at a time and add a few of your largest shims to each side of the gear. Place only that gear into the shell and screw the shell together. If the gear has some play up and down add some more shims, if the gear is too tight and does not spin freely take away shims or replace one with a smaller size.

Continue to do this until you cannot add or remove your smallest shim without allowing the gear to have vertical play or not be able to spin freely. Once this has been done to all three gears you will now have to shim the gears to each other.

 Shimming methods

There are several methods of shimming. Some techs like to shim the spur gear as close to the gearbox shell as possible. Others also like to shim the sector gear to the piston. Both of these methods are usually not recommended as they shim the bevel gear off of the spur or sector gear and not the pinion. Being as the bevel-pinion gear interaction is one of the highest stress points in a gearbox techs usually prefer the bevel to pinion method. Using this method you shim the bevel gear to the pinion and the rest of the gears to the bevel. Using this method the bevel gear can be accuracy shimmed to the pinion gear.

To shim gears to each other you can change the height of each gear by taking a shim from the top of the gear and placing it on the bottom (and vise versa). if you do not have enough shims to work with you can exchange a larger shim for smaller ones that add up to the same size.

If you have correctly shimmed your gun you should be able to hear a noticeable difference (though not always). Don’t be discouraged if you get frustrated while shimming, it took me a full day to learn how to shim my gun properly. Shimming is a must in all aftermarket builds and is a general recommendation for all guns stock or not.

AOE (angle of engagement)

AOE or angle of engagement is the angle at which the first tooth of the sector gear engages or meshes with the pick-up tooth on the piston. With most stock guns the AOE is anywhere between 5-20 degrees from vertical. This is important because like the bevel-pinion interaction the transfer of force is between two planes. This can be hard to understand without visuals.

When the AOE is not corrected the transfer of force is not linear with the movement of the the piston. The teeth on the piston or rack is designed to take stress at 90 degrees from the sector gear. When excess stress is put onto the piston from aftermarket parts the piston rack can be compromised causing damage to the piston.

When the AOE is corrected the transfer of energy is optimized and becomes more efficient. This also allows the piston to handle more stress and increases the reliability of the overall system.

AOE correction

Correcting the AOE is fairly simple. To do it all you need to do is move back the piston so that the sector gear engages in the correct spot. Moving the piston back can be done in several ways. A quick and easy way to do it is to place washers inbetween the piston and the piston head. However this give the screw holding the piston head and piston together less grip and may cause problems in high stress builds. One of the more common ways to correct AOE is to attach laser cut sorbothane or neoprene pads to the cylinder head inside the cylinder. This doesn’t compromise the piston and has the added benefit of shock reduction. Sorbo pads can be bought by Scatterplot on Brill armory.

When attaching the Pad to the cylinder head rough up the pad and cylinder head with sand paper on the sides that are glued together. Use some form of epoxy or cyanoacrylate (super glue) to form the seal. Sorbo pads can then be sanded down with a fine grit sand paper for fine adjustments. Also note that the addition of spacers or pads will reduce valuable air volume in long range builds.

Video tutorial

For a more in-depth video guide tutorial on the modifications listed in this guide please watch my M4 tuneup guide at-


I am always learning and expanding my knowledge of airsoft tech. If you notice anything wrong with this guide or have some helpful criticisim please don’t hesitate to let me know. It’s learning from my mistakes and others that helps guides like this grow. help me be more helpful to others by contacting me at or commenting on my website.

-Jon Kraatz






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