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Know Your Gearbox: A closer look at the standard M4 V2 gearbox

Know Your Gearbox MainLook at any Facebook group with a discussion of airsoft tech info and you will be overwhelmed with a tidal wave of information, some good, some marginal and some things that should be banished from the internet. If you are new to airsoft and more particularly how the gearboxes work then this article will help you understand the functions of your parts and what you should do if you would like to upgrade your gun.

Where to start? Well in this article we are looking at the internals of the AEG itself. We will not focus too heavily on the body style, BBs being used or the battery because that is an entirely separate can of worms. We want to focus on the driving force in your AEG.

The electric motor in your AEG is what drives the gun. It puts the E in AEG. Stock guns come with all sorts of motors from high-speed to high-torque. It can be an easy upgrade depending on what you want out of your gun. Be sure you get the correct length for your gun. There are three types of motors, short, medium and long. You can usually find this listed in the tech specs of your gun.

The gearbox shell is the main component making up your gearbox. This is what will house all the pieces that we are about to describe. These are made of pot metal or aluminum. They are precision created and can be different depending on the make and model of your gun. The most common is the V2 gearbox that we are looking at. It can be found in many M4s, Mp5s and even G3 series rifles.

Know Your Gearbox p1GEARS
Gear sets can really make or break your gun. Putting the right set in can really help improve its performance. You can create a very efficient gun that puts little strain on the motor and will pump out BBs on demand. Or you can install improper gears that will give you an awful screeching noise and can lead to the destruction of your gun. They offer many different cuts such as helical and standard cut as well as different ratios. The ratio will normally indicate if it is for a high speed gun, high torque/fps or standard gear set.

These small parts come in several sizes as well. They are a buffer between your gearbox shell and the gear itself. The bushings are typically the more durable of the two and are made from metal or plastic. The bearings are much more expensive and not quite as durable but offer greater speeds with less resistance. They all come in 6, 7, 8 and 9mm versions. Be sure to get the correct size for your gearbox shell. This is also normally labeled in the gun tech specs on the manufacturer’s website.

Shims are the small metal discs that are located on the sides of your gears. They come in many assorted sizes and thicknesses. These help keep UPI gears centered in your gearbox to keep them from rubbing on each other.

The spring guide is there to do exactly what you think, guide the spring. These come in both metal and plastic. They offer them with or without ball bearings that will help keep your spring from losing shape. One key difference to note when ordering a spring guide is to see what version of gearbox it is going in. Each one has a unique cut out for the spring guide.

The spring in a gearbox is what gives you the power. It is the one item that will affect the FPS of your AEG the most. Springs come in many sizes as well as linear and nonlinear versions. They are sold by the stiffness of the spring. An M90 spring will give you under 300 fps on average and a M120 will give you around 400 fps W/.20g BBs. The larger the number the harder it will shoot. It’s important that you match the spring with the proper gear set, motor and battery. Having the three parts match will make the gun shoot much smoother. High torque gears and motors with high fps springs. You get the idea.

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Piston & piston head

The piston is a crucial piece of the gearbox. It is also a piece that can fail quite commonly. These are typically formed from plastic or polymer with a rack of teeth inserted into them. While an all metal piston may be more durable. It may also end up costing you more by not failing.

The piston head is locked onto the front of the piston. Some have a ball bearing affixed on the inside to enhance the spring. The head itself has an O-ring that completes the air seal. The solid portion on the front can be metal or plastic, vented and non-vented. The vented heads will give you a better seal by forcing the O-ring to expand and fill the cylinder.

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Cylinder & cylinder head

The cylinder is the metal piece that holds the piston and is made out of metal either brass or anodized aluminum. The cylinder is the space that traps the air and the allows the piston and piston head to drive the air forward.

The cylinder head is made of polymer or metal. It will have a rubber pad on the back side and two small holes on either side to keep it inline within the gearbox. This is what will take the air that fills the cylinder and funnels it down into a small tube that will direct it into the hop-up.

The Nozzle is that will go over the tip of the cylinder head. These are made of plastic or metal and some have a small O-ring on the inside to help seal the air. This will attach to the tappet plate. The nozzle is responsible for loading the bb into the hop-up chamber and pushing a column of air behind it. It keeps the air from leaking out of the bottom of the hop-up unit. Different guns may have a different size of air nozzle so be sure to get the correct one for your model of gun.

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Tappet plate

The tappet plate is a very simple plastic plate. It sits under your cylinder and attaches to your nozzle as well as a spring that keeps it in place. When you fire the gun a small pin on your gear set will pull the tappet back. That in turn will pull the nozzle back, allowing a BB to come up into the hopup. The tappet will then spring forward and the nozzle will make a seal pushing all the air forward and the BB out.

The plunger assembly is part of the electrical system of the gun. This is the “on/off” switch for your gun. When you pull the trigger it will push a small piece into the center of two other metal contacts that will complete the circuit, causing your gun to fire. These are made of plastic to insulate the metal and keep it from shorting out.

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Plunger assembly

This metal piece is one of the only components of the gearbox that can be seen when a gun is fully assembled. Most are made from metal and have a spring to return the trigger to its home position when not firing. Although they were not available for years, you can now buy custom triggers to spice up the look of your AEG.

The selector plate is located on the outside of the gearbox. It sits in two grooves and controls both the safety lever and the cut-off lever. These are made of metal or plastic and some have a brass plate to run current for your gun as an added safety measure.

The safety lever is operated by the selector lever. This small plastic arm has a spring to keep it out of the way when you are firing. When the safety is engaged this bar will slide up and block the trigger from being pulled.

The cut-off lever is also manipulated by the selector lever and the gears in your gearbox. When a gun is put into semi-automatic fired the cut off lever will trip the plunger and the gun will only fire one shot. The cycle is continued when you pull the trigger again.

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Cut off lever

As you can see, every part of the gearbox is critical. Without one piece to this complex puzzle it can fall like a house of cards. Keep your investment in tip top shape to insure that you will have fun for years to come.