A Guide To Airsoft AEG Gearbox Parts and How They Work

November 16, 2014

Gear Box Shell: Also referred to as the ‘mechbox’ This is the case in which all of your internal parts are in. These shells usually split into two halves. Gear Box shells differ depending on the make and model of the gun, the most common gearbox shells are the Version 2 (V2) and the Version 3 (V3), other common versions are the Version 6(V6) and Version 7(V7). Less relevant versions are V1, V4, V5, V8, and V9. These less common versions are tailored to specific guns. For instance the V1 only works on the FAMAS, The V4 only works in the TM PSG-1, and V5 only works in the TM Uzi, etc.

Some of the more common shells specifically the 2 and the 3, come with different sized holes for bushings or bearings. The sizes are 6mm, 7mm, 8mm, up to 9mm.

Here is the list of gearbox shells with (some) of their designated airsoft AEG platform(s) in numerical order.

  • Version 1: FAMAS (TM and Cybergun)
  • Version 2: M4/M16 series, MP5, G3, ACR Masada, Sig Rifles, PDW Extended Version 2: SR-25
  • Version 3: AK series, G36 series, MP5 K, AUG
  • Version 4: TM PSG-1
  • Version 5: TM Uzi
  • Version 6: P90 and M1A1 Thompson
  • Version 7: M14
  • Version 8: TM Type 89
  • Version 9: TM AK74 MN
  • Tokyo Marui Next Generation: TM Next Gen Blow Back AEG airsoft rifles.
  • M249 and M60 Gearbox
  • VFC HK 417 Gearbox

Gears: These are the drive train components in your airsoft AEG these are what transfers power from the motor to the sector gear which pulls back the piston this is done. There are four gears in standard AEGs, the pinion gear (the gear on the motor), bevel gear, and sector gear.

Motor Pinion > Bevel > Spur > Sector > Piston rack

Each gear has a ratio in between the two giving you a final ratio, stock or standard ratio gears are 18:1 (18.72 :1) this final gear ratio is how many full pinion gear rotations it takes to complete one full rotation of the sector gear. As you can see here the gears themselves are a real torque multiplier, especially the spur gear. Lower gear ratios are classified as high speed, they go as low as 10:1, the lower the ratio the higher the speed, typically. The higher the ratio the slower the speed, torque gears (>18:1) make pulling back springs easier because they have more of a mechanical advantage, but they have a big draw back in the fact that they diminish on trigger response time and RoF. High Speed gears e.g. 13:1 are higher speed, but require more power to pull back a spring. This issue typically is not an issue even when pulling a heavy spring for an experienced tech using a high torque motor and a powerful battery.

Piston: This piece is a huge work force of the airsoft gun. These pistons are made out of different plastics from POM, Nylon, Poly Carbonate, etc. The piston will have 16 teeth. When the sector gear is spinning, it will pull the piston back. Inside the piston is the AEGs main spring, so when the piston is pulled back it compresses the spring. The spring then gets released when the sector gear has completed its cycle and the sector gear’s teeth are no longer contacting (pulling back) the teeth on the piston. The teeth on the piston can vary in material as well from plastic teeth to a full metal tooth slide in and removable rack or half metal rack, but regardless the last tooth on the piston must be metal because it is the tooth under most stress. Piston Head: The piston head is attached to the front of the piston by a screw. On the piston head is an O-ring that rests in a slot, this as the piston assembly (piston and head) goes forward that O-Ring assists in sealing the existing air in the cylinder, the piston head then pushes that air forwards all the way to the cylinder head and so on. Some piston heads feature ports, supposedly when air goes through the ports it pushes the O-Ring outwards expanding it and creating a better seal, this is not entirely a proven fact.

The most common piston head material types are POM and Aluminum. POM is often favored as it is lighter than the bulkier aluminum heads.

Ball Bearing Piston Heads: Most quality piston heads come equipped with ball bearings; these ball bearings are placed inside the piston to help the spring twist when it gets compressed. These bearings are not entirely needed if you already have a ball bearing spring guide, it is really only necessary to have bearings on one side of the spring.

Motor: The power plant of any AEG. For TM style airsoft AEG’s there are 3 types of motors Short, Medium, and Long the only reason is because certain guns are shaped a certain way, there is no difference in performance it is merely the size of the armature. The motor can be broken down into several parts the armature, the Can – with attached magnets, the End Bell, and the brushes. When looking for a motor it does not have to be expensive, it is good to change out your stock motor mainly because they come with ferrite magnets. Neodymium magnets are what you should be looking for, almost all aftermarket motors come with these magnets.

Airsoft AEG Gun Motors are classified into two types high speed and high torque, high speed motors have less windings (lower TPA) wound around the armature increasing RPM, high torque motors have more windings (higher TPA) increasing torque and efficiency. Because of the higher TPA count higher torque motors will run cooler than high speed motors. In addition to that speed motors can really only run well or better than torque motors on a weak spring.

Bushings / Bearings: There are 6 bushings (and/or bearings) in the airsoft guns gearbox, these are on either end of the gear axles. The bushings keep the gear axles in place so the gear can rotate as it needs to.

The size used depends on the mechbox shell, either 6mm, 7mm, 8mm or 9mm.

  • Bushings: Completely solid pieces tend to be more reliable (depending on bushing material). Bushing size does not affect reliability.
  • Bearings: Reduces friction allows gears to spin with ease. Bearings size does affect reliability, the larger the bearing the less prone they are to breaking/ blowing out. It should be noted that ceramic ball bearings are extremely durable and are much stronger than regular metal ball bearings.

Spring Guide: The guide rod of the main spring fits firmly in the gearbox shell. The spring guide is held onto by both sides of the gearbox shell and sometimes with a screw going into through the back of the gearbox shell. It is preferred that the spring guide have ball bearings and/or washers on the back of it so the spring can rotate when compressed.

Main Spring: The spring that drives the piston assembly forward into the back of the cylinder head which compresses the existing air in the cylinder. All aftermarket springs come with a rating, the rating correlates with the stiffness of the spring. Stiffer springs will compress the available air in the cylinder much quicker giving you a higher airsoft gun FPS. The ratings are typically measured in Meters/Second, from 85 M/S ~210 M/S.

Anti-Reversal Latch: After the airsoft gun cycles it is natural for the gears to want to torque backward. The anti-reversal latch is there to make sure the gears do not rotate backwards. The Anti-Reversal Latch (ARL) contacts the bevel gear on a number of gears on the bevel gear, when the bevel gear begins to rotate backward; the ARL catches the reversal gears on the bevel gear and stopping it.

Cylinder: As discussed briefly when talking about the Main Spring, the cylinder is where existing air gets used to propel the BB, the piston head drives the air through the cylinder head and so on. Depending on what barrel and what weight BB you are using only a certain amount of volume is needed out of the cylinder to propel the BB all the way to the end of the barrel. This is why cylinders are often ported. A lot of TM style airsoft cylinders are rated by a certain type, these cylinders are made for certain guns with specific barrel lengths, but truth be told it really depends on what weight BB the user is going to be running.

**For instance take an airsoft M4 cylinder (standard port), that cylinder will work fine when running a .20 gram BB and mostly likely a little bit heavier than that. However through a .36 gram BB in there, you will need a higher volume of air to propel that heavy weight round to the end of the inner barrel, most likely a full (non-ported) cylinder.

Cylinder Head: The airsoft cylinder head is attached to the end of the cylinder. It’s the cylinder heads job to redirect all of the air that was compressed in the cylinder (that was compressed by the piston head) into a little tube to be transferred to the attached air nozzle into the barrel group. It is important to ensure a good seal between the cylinder and cylinder head.

Air Nozzle: The Air nozzle covers the cylinder head tube and moves back and forth over it. The air from the air flows through the cylinder head into the air nozzle the air nozzle releases all of the air that has been made into the bucking lips which seals the air between the hop up and the barrel.

**Note this nozzle is attached to the tappet plate, when the tappet plate is in its most forward position the air nozzle is as well, the tappet plate is also timed with the sector gear so when that gear just released the piston, the tappet plate with attached air nozzle returns forward, in that most forward position the air will be released at the correct time.

Tappet Plate: The Tappet Plate is spring loaded inside of the airsoft gearbox shell in its most relaxed position it is all of the way forward and attached to it is the air nozzle. The fin of the tappet plate rides along a knob on the top face of the sector gear, as the sector gear rotates backwards it pulls back the tappet plate which pulls back the air nozzle allowing a BB to be chambered into the airsoft AEG.

Trigger Switch: A standard trigger switch (with no MOSFET) is the on and off switch for the motor, it consists of a sliding contact (male contact, or trigger trolly, etc.) That sliding contact, when the trigger is pulled, makes contact with the female contacts, the female contacts consist of two contacts the side, the male contact one side is soldered to the Positive motor wire, and the contact on the other side is soldered to the positive battery wire. The trigger is attached to these sliding male contacts if not attached there is something in between them so whenever it is pulled the motor will spin causing the gun to fire.

Wire: Wire is the gateway between your airsoft motor the battery and the trigger switch. It transfers electricity from your battery to the motor via the trigger switch. Wiring configurations vary from gun to gun and will be different if you are running a MOSFET. Common AEG wire gauges are 18AWG and 16AWG, you can purchase aftermarket silver plated wire as well and run as low as 14 AWG, but wire that thick is not necessary.

Cut-off Lever: The cut off lever does exactly what it sounds like. When you select the semi auto position your gun the cut off lever gets moved, how it gets moved varies from gearbox to gearbox. When in engaged, it will be in the semi-auto firing position and it bumps the trigger trolley off of the trigger so it no longer makes contact and stops firing. The cut off lever is engaged by a cam on the sector gear.

Selector Plate: The selector plate rides along the side of the airsoft gearbox shell, when the selector switch is turned the selector plate moves the cut off lever, so your airsoft AEG can fire in semi and fully automatic firing modes. The selector plate serves other purposes, but it all depends on the type of airsoft AEG that you have.

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