Gearbox Internals
What all those little parts do
The gearbox is a very complex piece of kit and it is quite easy to get overwhelmed the first time you take one apart and have to go find the spring guide as it shoots across the room leaving the rest of the gearbox parts a scattered mess on your table. Don't worry, we've learned a lot over the years and want to help you better understand what all those little things in there do and why they are each important to the proper functioning of the gun as a whole. For this article, we will use a stock type 2 gearbox as they are the most common. The differences between the different gearboxes are relatively minor and I will mention them where necessary. Many of the parts pictured below came from a stock rifle so you can still see a little bit of grease left on them. Chances are they will look very similar to the parts in your own rifles.
Main Spring
This is the part of the gearbox that gets the most attention. When the spring pushes the piston assembly forward, the air trapped in the cylinder is what blows the BB down the barrel. Naturally, the stronger the spring, the faster the BB will go. That isn't all there is to it though. If the air leaks around the piston head, or out of the cylinder, the BBs will not go as fast. That is why upgrades typically involve a lot more parts than just the main spring, as you will see below. Different companies have different ways to rate the strength of their springs and some make more sense than others. Systema, for example, uses a meters per second rating for their springs. This means an M100 is expected to shoot about 100 meters per second. That is roughly 320-330 FPS. PDI on the other hand uses a % system that as far as I can tell just means how much harder the spring is to compress versus a stock spring. These are a little harder to figure out how fast the gun will be shooting, but it will let you know immediately how much more stress it is putting on the rest of the gearbox.
Piston
The piston is the part the gears pull back to compress the spring. Those teeth on the bottom are all that stands between a gun that shoots and a gun that makes a terrible noise. That is why it is important to get a piston that will hold up better with stronger springs. They make pistons out of lots of materials. Nylons, and aluminums being the most popular due to their light weight and relatively high strength. Some companies make nylon pistons with several metal teeth built in for more durability and some of the pistons can get quite expensive. Pistons in upgraded guns are usually one of the first things to wear out so stronger and better made pistons are an important part to consider when thinking of upgrading.
Piston Head
The piston head bolts onto the front of the piston and has an o-ring around the sides so it can get a good seal with the cylinder. If the cylinder and o-ring do not match up well, you will not get a good seal and your gun will not shoot as well as it should. If the o-ring wears out you will notice your gun making weird noises and a big drop in range. Same thing if the piston head ever comes unbolted but even more noticeable. Some piston heads focus on being light weight so the piston and spring can decompress quickly and give you higher velocities due to an efficient design. Others prefer to have a thick rubber bumper on the front and these are good for insane modifications that put a lot of stress on the gearbox shell. Many will have a thrust bearing on it as well but I tend to frown on those.
Spring Guide
A good spring guide is one of my favorite parts. Since it is a stationary part, it can be as heavy and durable as you need without being a disadvantage. The spring guide on the right is a very basic metal spring guide from an entry level rifle with no bushings or bearings. The one on the left is a good smooth metal with a thrust bearing on it. This is important because as the spring is compressed, it tries to twist. If it can not twist, it expands to the sides and can rub or jam on the inside of the piston. Naturally, this leads to poor shooting performance and the occasional lock-up of the entire gearbox. The thrust bearing on this spring guide helps keep that from happening. Plus, because the thrust bearing takes up a little bit of space, it puts a little bit more of a load on the spring so you get 5-10 more FPS out of the gun just from that part alone. This is a good place to have that heavy part as opposed to the piston head that you want to be as light and agile as possible. I have seen upgrades that have the piston head with a thrust bearing as well as a spring guide with a thrust bearing. When using some springs that are very thick, it does not allow enough room for the piston to fully compress when the gears are trying very hard to push it back. This can cause the gun to lock up or, worse, strip the teeth on either the piston or a gear. This is why I had to clip a few coils from the thick Systema main spring in the picture above. It is still a good idea to have a thrust bearing to let the spring function easier so the spring guide with a thrust bearing is the best place for it. Having that extra weight on the piston head just slows down how fast your piston can cycle and puts more stress on your gearbox as it is more mass slamming against the front of the gearbox hundreds of times a minute. This can cause your gearbox shell to crack and is an expensive part to replace.
Cylinder
Different manufacturers offer various types of cylinder but the basic job of keeping the air in remains the same. While some manufacturers offer cylinders made of steel, anodized aluminum, teflon coating, or with a ribbed exterior, most are just brass. Brass cylinders are smooth and cheap but tend to tarnish like the one pictured. While it is not pretty, you don't ever see it unless you take the rifle apart to get to the gearbox. There are three main types of cylinder. The Type-0 cylinders like the one pictured are pretty basic. A solid metal tube. Nothing special. Type-1 cylinders have a small hole near the back to allow excess air to pass through it. Type-2 cylinders have the hole a little further forward. This is to make the gun more efficient for different barrel lengths. I can write a long article about it but it basically boils down to long barrels need a Type-0, slightly shorter barrels use a type-1, and even shorter barrels use a type-2. There are bore up cylinders for incredibly long barrels that require replacing the cylinder head, and piston head at the same time so they are usually sold in kits.
Cylinder Head
The cylinder head caps off the front of the cylinder so the air is forced through the smaller hole in the center which goes in the nozzle. A small o-ring goes around the outside of the unit for a good seal with the inside of the cylinder once it is inserted to the right height. Holes in the side of it allow posts in the gearbox to make sure it stays secure. Good thing too because if this part managed to get out of line, it would bind the tappet plate and move the nozzle out of position. There is a little bit of dense foam padding on the inside to help reduce the stress of the piston assembly slamming into it all day. This part is made from nylon most of the time but higher quality parts are available in aluminum or brass and may have two o-rings around the outside as well as a smoother channel for the air to go through for better efficiency.
Sector Gear
Not only is this the gear that pulls back on the piston but it also keeps the rest of the cycle happening in the right order. There is a bar on the right side of it that moves the tappet plate and nozzle as it spins, and a cam on the left side (not pictured) that moves the disconnect lever. This one part handles all the timing of the gun. Note the big strong teeth on only one section of the gear to pull back the piston as it turns and how it is smooth on the other to allow the piston to decompress once the teeth are out of the way.
Spur Gear
The spur gear is there to allow gear reduction between the slower moving sector gear and the faster spinning bevel gear. This part is also where the gears get most of their torque. In gear sets meant to provide even more torque, you will see fewer teeth on the inner section for even more gear reduction. This usually means you have to have a bevel gear designed to go with it. The opposite is true of high speed gears. This is the main reason why different ratio gears are sold as sets so you do not get confused trying to figure out which gears work together. In guns with nylon bushings, the spur gear is usually the first part to wear out. I will explain that in more detail in the bushings section below.
Bevel Gear
The bevel gear serves three purposes. The small teeth on the inside turn the spur gear while the large teeth on the outside are cut at an angle to allow it to mesh up with the pinion gear on the motor. This is necessary because the motor is mounted at a 90 degree angle compared to it and if they aren't at the right angle, bad things happen. It is also important to make sure that the motor is adjusted to the right height so the pinion on it can mesh up with the bevel gear properly. You can find more info on how to do that on the Gun Care Tips page. The bevel gear also has at least two notches cut around the middle to allow the anti-reversal latch to stop the gun from decocking when you release the trigger. I will explain more about this in the anti-reversal latch section below. The gear pictured here has four notches and I have seen as many as five. I have never noticed a performance advantage for having more notches than two so do not worry about not having enough.
Anti-Reversal Latch
When you stop shooting the gun, if the main spring has been compressed any, it will push the piston forward again and spin the gears in reverse. This is bad because it will allow multiple BBs to get into the barrel ensuring that the next shot is horrible, but also to keep the motor from spinning in reverse doing bad things to the electric system. If this is not put in right, the gun locks up, sounds like it is trying to cough, scream, or any number of bad things. It is also the most annoying part to put back in because of the way the spring that keeps it in place tries to do exactly the opposite when you are putting the gearbox back together. Grrr.
Bushings
There are three main types of bushings. Metal bushings, nylon bushings, and ball bearings. Granted, the bearings aren't necessarily bushings, but they do the same job so I'm going to put them here anyway. The nylon bushings offer very low friction and allow the gears to spin without very much fuss. They are also very cheap and easy to make so you find these in most rifles. The problem is that they can wear out a lot faster than the metal bushings and allow the gears to get out of line and strip. Not good. Metal bushings are almost mandatory on a gun if you want it to last a long time. You may be wondering why they even bother with bushings at all if the gearbox is metal, why not just build the bushings into the gearbox and make it one piece. Well, even bushings wear out eventually and they are a lot cheaper to replace than the entire gearbox shell. The ball bearings offer the advantage of low friction to the spinning gears while offering more support than the nylons. Guns with ball bearings are known for their higher rates of fire, and quieter, more efficient operation but are not as reliable as solid metal bushings. When a ball bearing breaks, it takes your gears with it. Fantastic for low to moderate powered springs, but discouraged for high powered guns.
Shims
Shims make sure that the gears aren't rubbing against the sides of the bushings and, more importantly, that the gears all line up against each other properly without binding. A sloppy shim job can ruin your gears. A good shim job makes sure that they flow nicely without binding.
Nozzle
The nozzle slides over the extended part of the cylinder head. It pushes the BB into the hop-up chamber and bridges the gap between the cylinder head and the hop-up. A good nozzle has to fit tightly around the end of the cylinder head to stop air from leaking and make sure all the work the rest of your gearbox is doing goes where it is supposed to. Some manufacturers produce nozzles that have little o-rings on the inside to make sure they get a better seal.
Tappet Plate
The tappet plate is conneced by a spring to the front of the gearbox and holds on to the nozzle. The post on the right side of the sector gear pulls back on the rear arm of the tappet plate which moves the nozzle allowing a BB up into the hop-up chamber. The spring on the front of the tappet plate pulls it back forward again creating a seal between the cylinder head and the hop-up bucking. It is quite ingenious really.
Selector Plate
When the selector plate is moved all the way back, it makes a connection for the trigger switch assembly so when you squeeze the trigger, it can start shooting. If you think of the trigger switch assembly as a light switch, then the selector plate is like the breaker switch in your basement. When the plate is to the rear, it also holds the cut off lever out of the way. Now the gun can fire full auto. When the selector plate is in the middle, it still makes the main connection for the trigger switch assembly but also allows the cut off lever to do its job. You can see the small spring at the top that pushes the lever back where it needs to be between shots. If that spring gets lost the gun will fire full auto when in semi auto mode. This can be a real pain because you can see how small and easy to lose that spring is. When the selector plate is moved all the way forward, it stops making a connection for the switch assembly and pushes against the safety bar seen at the bottom left. For a mostly flat piece of plastic, the tappet plate has quite a job.
Safety Bar
When you move the selector on your gun to the SAFE position, it pushes the selector plate all the way forward. When the selector plate is moved all the way forward, it pushes the safety bar up so that it stops you from squeezing the trigger. That's all. As long as the selector plate is moved all the way forward and disables the circuit for the trigger switch assembly inside, that should be all you need to keep from firing the gun in SAFE mode, even when you squeeze the trigger. Then why have it? Amazingly, when Tokyo Marui first designed the gearbox, the engineers put so much time and effort into the little details to make sure the guns were as realistic as possible. This includes not being able to pull the trigger back when the selector is set to SAFE. How cool is that? It's the attention to those little details that make me smile. Also, you can see the trigger in this picture. Obviously you can tell this gearbox came from an M4 by the shape of the trigger. An MP5 uses the same type of gearbox but uses a trigger with a different shape. A type 3 gearbox would look quite a bit different here and has more moving parts in the trigger area.
Trigger Switch Assembly
Basically, the on off switch for your gun. The trigger moves the trigger block forward so that it completes the circuit from the battery to the motor and back again. A small spring pulls the trigger block back out again when you let go of the trigger or when the trigger disconnect lever kicks it out in semi auto fire mode. Different trigger switch assemblies are made better than others, mostly because of the quality of the wire, the finish on the leads, or how easily the plastic that holds the assembly together melts. I have seen a lot of issues from having a trigger switch assembly burn out or melt. If you are really hard on your gun and like to shoot full auto bursts for more than three or four seconds at a time, you can expect to replace this part some day. The picture here does not really make it easy to see what I'm talking about. It is very hard to take a picture of it that makes sense. If you look at the picture of the safety bar above, you can see a closer picture of the main trigger switch assembly above the trigger.
Cut Off Lever
This funny looking bit is pushed down by the cam on the left side of the sector gear and lifts up on the trigger block so the gun stops firing after it has made a single shot. The small arm that sticks out the back goes through a slot in the gearbox where it connects to the small spring you saw in the picture of the selector plate. When the gun is in full auto mode, it is held out of the way allowing the gun to keep firing until you release the trigger. You see that little spring there
Gearbox Shell
The skeleton of the gearbox and the only thing keeping everything together. There are many configurations (this one is a type-2) but the differences are minor between the different styles. If you are unlucky, the gearbox itself can crack and needs to be replaced. This is more common with stiffer springs, bad piston heads, and very cold weather. The gearbox shell plays the biggest factor in how much you can upgrade your rifle before it is certain to destroy itself in the process.
Motor
There are lots of different kinds of motors. The three different sizes are Longs, Mediums, and Shorts like the one pictured here. Different types of gearboxes require a different size of motor. For example, the type-2 gearbox requires a Long motor. Each of these also has different models offering higher torque, or higher speed. Often the manufacturers will claim both but only personal experience in using the different types can reliably tell you where they all sit on the totem pole. Basically, keep using the motor the gun comes with until it dies and then weigh your options carefully if you want to upgrade to something else. The motor will not increase how far your gun shoots, just how often. All it does is cock the gun. The beefier motors with more torque are good for guns that have stronger main springs in them, while the high speed motors are good for guns with normal springs and want to shoot more BBs per second. Think of it as driving a car up a hill. The stronger the spring, the steeper the hill. The high speed motor will be like trying to drive up the hill in top gear while the high torque motor is more like 3rd gear.
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