AEG Guts and You

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c/o Airsoft Retreat:

http://www.airsoftretreat.com/reviews/showproduct.php?product=67&title=-how-to-3a-a-guide-to-your-mechbox&cat=49

(Archived: https://web.archive.org/web/20151005074813/http://www.airsoftretreat.com/reviews/showproduct.php?product=67&title=-how-to-3a-a-guide-to-your-mechbox&cat=49)

By popnfresh wLcH Originally Published: July 2005

Disclaimer

Airsoft Retreat Forums and the author of this article assume no responsibility of problems stemming from use of this guide. It is YOUR decision to upgrade your gun, not mine. If the upgrade fails, it is not my fault, as I did not upgrade the gun. The methods portrayed in this article have been used by myself and many other upgraders from all over the country, and are a tried and true set of techniques. Any problems that occur can be communicated to the ASR forums to get a diagnostic from its members so you can buy any necessary parts to fix your gun and install them.

Introduction, the Beginning of a New Revelation in ASR AEG's

AEG’s. While they may be a cool way to get automatic fire in an efficient manner, they all run on a bunch of parts connected together and enclosed in a tight shell: the mechbox. Probably the single most terrifying experience with any airsoft gun is opening the mechbox for the first time. In the mind of a person new to the guts of an AEG, the main reason they want to have a shop do their upgrades is because they don’t want to break their gun. Unfortunately, upgrading at a shop comes at a high price, because labor usually runs on an hourly rate, and shipping to the shop and back is quite a hassle and rather expensive.

Being rather experienced in the inner workings of the mechbox, I decided to step in and help people assuage their fears of opening their mechbox, so they don’t have to go through that trouble. Besides, shops usually have huge queues of guns they need to upgrade, solely because the person wants the upgrade, but is too afraid to do it themselves. This huge queue forces the shop to work fast, and working to fast produces problems.

So, without further ado, popnfresh presents to you Mechbox 101, outlining everything you need to know about why your AEG works, as well as how to do some simple upgrading.

I. Overview of the Mechbox (outside)

First, to even get to the mechbox, you will need to strip your gun completely so you can dislodge the mechbox from the gun’s body. This usually means removing the pistol grip, a few pins, a few miscellaneous parts, and sometimes, the stock. Once the mechbox is dislodged from the body, you will notice several distinct sections of the mechbox, and what they contain is good to know before you open the thing up so you don’t have a spring guide flying across the room.

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Before starting, I also recommend working on a clean, flat, and well-lit surface, so if any small parts get lost, they can be more easily found. Also have in front of you a set of flathead and Phillips head screwdrivers, a set of Torx wrenches (more specifically, a T10 driver), some Allen wrenches, some white lithium grease, and a pair of needle nose pliers. Depending on what you’re going to be doing in the mechbox, different tools will most probably be necessary.

Now, distinguish the two sections of your mechbox. The upper section will be wider than the rest of the mechbox, contains a long, brass cylinder, and has a small plastic piece protruding from the end. The lower section will be flat, will contain the trigger, and has small, metal/nylon (depending on if you’re working on CA/ICS/AE or TM) bushings protruding from the center of the mechbox. We’ll talk about the upper section first, and the parts it contains.

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The upper section is home to the cylinder/piston/spring assembly. This is where all the compression takes place in the mechbox, thus allowing the gun to fire BB’s at velocities of 280 FPS and higher. When the gun is fired, the motor spins, the gears spin, and the piston is pulled back against the weight and pressure of the spring. When the gears reach the end of the piston’s teeth, the piston is released against the full strength of the spring. The piston then shoots forward through the cylinder. The o-ring, which is housed on the piston head, expands to the diameter of the cylinder and compresses the air. This compressed air is then channeled through the cylinder head and the air nozzle and then to the BB, which is shot through the barrel.

Now, we move on to the lower mechbox, which houses all the electrical equipment and the three gears which make the gun run. The trigger unit is home to many different small parts, but it is not necessary to mess with certain parts down there on a TM V2/V3 mechbox basic upgrade. Depending on how your gun is wired, there may be wires running out the front of the trigger unit, or out the back of the mechbox. The wiring that extends out the back or front (depending on what gun the mechbox is made for) will be connected to the battery. The two wires coming out the bottom of the mechbox are what feed power to the motor.

When the trigger is pulled, small contacts inside the trigger unit feed energy from the battery, through the selector plate (on V2 and V3 only) and into the motor, which engages the gears, pulls the piston back, and releases the piston against the pressure of the spring, causing compression to shoot the BB. If any of these wiring connections loosen or don’t engage at all, the gun will not work, or work very sporadically.

During the firing process, the tappet plate, which is connected to the air nozzle, is pulled back by a small nub on the sector gear against the weight of a very small spring. This pulls back the air nozzle, which allows a BB into the chamber. When the nub disengages the tappet plate, it is released under the tension of the spring, which allows the air nozzle to move forward and chamber a BB, just in time to channel the compressed air from inside the mechbox.

V6 Note:

There is a complex set of wiring connections on the outside of the V6 mechbox, thus allowing it to function without a selector plate. In order to open the mechbox, this wiring compound will need to be removed by unscrewing all of the screws on the black plastic that holds it to the mechbox.

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The positions of the bushings on V2 and V3 mechboxes will run from the motor in a slightly bent line to just behind the cylinder. Take note that on V2 and V3 mechboxes, the bushings on the left side of the mechbox will be obstructed by the selector plate; you will not be able to see them all. On V2 and V3, the bushings will be perfectly round, with a small hole in them to house the gear axles.

In a V6 mechbox, the bushings are in more of a backwards L-shape, and are partially obstructed on the right side by the wiring assembly. Also note that the two bushings that hold the spur gear (the middle one) in place are the oval shaped ones. Remember that the hole is on the forward side of the bushing. If the bushings aren’t in the correct position, the gears will not fit together properly, and the mechbox shell won’t even close.

Last on the outer overview are the motor cage units on the V3 mechbox, and the complete location of all the screws on the mechbox.

In order to open the mechbox, you will need to remove the motor cage unit on the V3, which will allow the two halves to open up. Set it aside for later, since it’s much easier to test the gun with just the motor instead of having to put the entire gun back together.

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The upper mechbox on a TM V2 will contain 4 screws that line the upper “rim” of the mechbox. These screws will be in this orientation: Phillips head, Torx, Torx, and Phillips head. On a Version 3 or 6 mechbox, there will be an aluminum stopper rail that is slightly shorter in length than that of the mechbox.

CA/AE/ICS Note:

On the Version 2 mechboxes of all these brand names, ALL of the screws are Phillips head. Don’t bother looking for Torx screws on your mechbox.

The lower mechbox contains 4 more screws (or the only 4/5 screws on V3/V6) located on the back of the mechbox; one on the bottom, one near the trigger unit, and one near the air nozzle. The order of Torx screws on the V2/V3 goes as follows: Torx, Phillips head, Torx, Phillips head.

V6 Note:

The orientation of the screws on your mechbox is slightly different than those of the V2 and V3. Aside from having 5 screws like the CA mechbox, the order of Torx and Phillips head screws are as follows: Phillips, Phillips, Torx, Torx, Torx. The second Phillips head screw holds the lower wiring assembly to the mechbox.

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CA Note:

CA and TM V6 mechboxes come with 5 screws on the lower mechbox unit. On the CA mechbox, there is an extra screw in the lower gearbox where the bevel gear resides.

Also, begin to take note of the various holes around your mechbox and what they contain. It’s truly helpful when putting the mechbox back together, since you’ll know what you’re working with.

II. The Parts and What They Do

This section is an in-depth analysis of the various parts in the mechbox, and what they do. It is better to learn about these parts now, before you open the mechbox, so you know what part is which. We’ll start with the upper mechbox again:

IIa. The Cylinder

The cylinder is the long, brass tube that occupies about one third of the area in the upper mechbox. The cylinder is one of the main parts of compression in the mechbox; it is coupled with three other parts in order to maintain perfect air compression.

Depending on the gun your mechbox was made for, the cylinder could be any one of five types: 0,1,2,3, and 4. All these types have small holes in them that are placed in different places, so as mentioned, be sure to note where the hole was for your mechbox. Again, if you have a Type 0, or “no-hole”, cylinder, don’t worry about placing the hole in the right place, since there is none.

IIb. The Cylinder Head:

The cylinder head is one of the three parts couple with the cylinder to maintain air compression. The part is easy to recognize, as it is the only part that is physically stuck in the cylinder. However, it is easily removed from the cylinder; it should simply slide right out of the cylinder, once it is removed. Looking at it more closely, you will see a ring that encircles about ¾ of the circumference of the part. This ring is also home to two small holes, which are fitted into small nubs on the mechbox’s inner wall. These keep the part from spinning every time the gun is fired, as to save the tappet plate from being misaligned.

The back of the cylinder head is home to small hole and a black, rubber surface which absorbs the impact from the piston head. The small hole on the back of the cylinder head is the first step in channeling the air for “export” down the barrel. On the front end of the cylinder head is a small protrusion where the air nozzle is placed.

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This picture contains both the cylinder and the cylinder head.

IIc. The Air Nozzle

The second of the three parts directly involved for compression, the air nozzle is the part solely responsible for the chambering of a BB and the channeling of air behind the BB every time the gun is shot.

Visually, it is the relatively small piece of long, thin plastic that slides onto the cylinder head. Depending on what gun the nozzle was made for, it could be of various lengths to compensate for the space between the mechbox and the hop up unit. On the back of the air nozzle is a small “rim”, which is where the tappet plate holds the air nozzle and keeps it from being shot out of the mechbox.

IId. The Tappet Plate

The tappet plate is attached directly to the air nozzle. It is a longish black piece of plastic that has an “arm” near the back that engages the sector gear and a half-circle slot in the front that the air nozzle is fitted into. There is also a small notch on the bottom near the middle, where a small spring resides. When removing the tappet plate from the mechbox, be sure to catch the spring. If it flies out of the mechbox and you lose it, the tappet plate will not function correctly when the gun is shot.

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This picture contains both the air nozzle and tappet plate.

ICS Note:

The tappet plate in ICS mechboxes has been reported to be very brittle when compared to TM and Guarder tappet plates. It has a rather well-documented history of snapping in various places for no apparent reason. It may be a good idea to pick up an aftermarket tappet plate if your gun hasn't had this problem.

IIe. The Piston:

The next part in the upper mechbox is the piston. The piston’s body is home to a bunch of small teeth that engage the sector gear, which is, in turn, pulled back against the tension of the spring. There may also be one reinforced metal tooth, which takes most of the stress from the spring as the gears disengage the teeth and allow the piston to fire forward. Also note the “guide rails” on the side of the piston. Those fit onto the small rail on the mechbox wall, to keep the piston’s teeth in place.

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The piston is directly coupled with the piston head, but the actual piston has almost nothing to do with the actual process of compressing air. It is classified more as a “flat-toothed” gear rather than an actual piston. The piston is more responsible for taking the stress from the gears and spring and positioning the piston head to maintain the perfect the air compression necessary for a consistent gun.

IIf. The Piston Head:

The third and final part coupled with the cylinder for compression, the piston head is attached to the front of the piston via a bolt and nut assembly to keep it from wobbling. Depending on whether you have a stock or upgraded piston head, there may be several small holes on the front side of the piston head. Air goes in through these holes and helps expand the o-ring on the piston head to fully engage the cylinder’s wall.

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The piston head also comes in a bearing version, which helps relieve some of the stress from the spring by making the spring coil and uncoil slightly as the gun is shot. This results in a slightly more consistent air compression rate. The bearings also act as a small spacer, and can increase FPS very slightly, in the 5-10 FPS range.

CA Note:

The CA piston o-ring has had a troubling past, being slightly too small to fully engage the cylinder on a highly upgraded spring. This problem could range from a small, 10 FPS drop, to even a 100 FPS drop. You can easily replace it with a larger one from your local hardware store.

IIg. The Spring Guide:

The spring guide is the piece in the back of the mechbox. It may or may not have small metal tabs on the end. If it does, make sure you note where they go, or the mechbox will not close properly. The part has a long extension that does exactly as its name sounds, guide the spring. It keeps the spring from bending places it shouldn’t, like into the gears or rubbing on the inside of the piston.

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Like the piston head, the spring guide also comes in a bearing variant, which performs the same function as the bearings on the piston head. Again, the bearings act as a small spacer, increasing FPS by about 5-10.

IIh. The Spring:

The spring is the driving factor behind speed in your mechbox, but choosing an extreme upgrade spring (like an M150) without reinforcing the necessary parts will destroy your mechbox faster than you can say “What the...?” Trust me; you’ll know which spring I’m talking about.

TM/ICS/AE Note:

Replacing the stock spring in your mechbox will require taking apart the piston head to unlatch the spring from inside.

The lower mechbox half doesn’t require nearly as much description, but the parts down here break most often, and it's good to know what you’re dealing with.

IIi. The Gears:

The three gears are the main driving force of mechanical energy in the mechbox. They take almost all the stress from firing, and have a pretty complex set of teeth that all have their purpose. The three gears, in order from the bottom of the mechbox, are the bevel gear, the spur gear, and the sector gear.

The bevel gear directly engages with the motor, and becomes the first step in increasing torque. There are three sets of teeth on it; the long-circular rack of teeth that engages the motor, the “spur-ish” teeth that engage the spur gear, and the less obvious, wave-like teeth that engage the anti-reversal latch. It is placed in the mechbox with the large, flat, circular side up against the right wall of the mechbox.

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The spur gear has two sets of teeth; a large circular rack that engages the bevel gear, and the much smaller rack engaging the underside of the sector gear. It is placed with the large, flat side down, against the left wall of the mechbox.

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The sector gear is the gear that directly engages the piston, and is the final step in creating the amount of torque necessary to pull back the spring. It has a large, and clearly defined rack of the teeth that engage the piston, and a small rack on the underside that engage the spur gear. It also has a small nub on the inside of the first, larger rack that engages the tappet plate and allows it to function. There is also a small egg-shaped nub on the underside of the sector gear, which engages the cut-off lever, which is necessary to retain proper semi-auto function.

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IIj. The Anti-Reversal Latch:

The anti-reversal latch is an extremely small, albeit extremely important, part which is necessary to keep the gearbox from "un-cycling" after a long stream of automatic fire. The part has a post like object that fits into one of the holes in the mechbox wall, and a small triangular protrusion attached to a small spring.

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The latch resides on the underside of the bevel gear, which engages the wave-like teeth on the gear. After the gearbox stops cycling, the spring-loaded anti-reversal latch hits one of those wave teeth and stops the gearbox cold. Seasoned upgraders also find it to be the hardest part to place in the gearbox, since the small spring that allows it to function doesn't allow it to stay put.

Note:

If you are not sure if the anti-reversal latch is actually serving its intended purpose (i.e. keeping the gears from un-cycling), it's probably a good idea to disengage the latch before the mechbox is even opened. You can do this by sticking a long, thin screwdriver in the hole where the motor usually resides, and pulling the latch off the gear. If it was in fact holding the gears, you will hear a "whirr" and a "thunk" noise from the gears and piston head, respectively. Don't worry about it; this will make it much safer to actually open the mechbox.

IIj. The Anti-Reversal Latch:

The anti-reversal latch is an extremely small, albeit extremely important, part which is necessary to keep the gearbox from "un-cycling" after a long stream of automatic fire. The part has a post like object that fits into one of the holes in the mechbox wall, and a small triangular protrusion attached to a small spring.

IIk. The Bushings:

The bushings are easily one of the more forgotten parts in the mechbox. However, seasoned upgraders make them a necessary upgrade when upgrading the spring in a TM mechbox. The bushings are very simple, circular pieces of nylon/metal that have very small holes in them. These holes house the gear axles, and keep them in place. The bushings take all the stress and friction from the gear axles turning, and are thus, rather important.

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Bushings come in a few variants: regular, laminate, and bearing. Regular bushings are just metal bushings, and perform the same function as stock bushings. Laminate bushings are known as the “half-assed approach” to upgrading the bushings, since they have a layer of nylon inside the bushing. This kind of defeats the purpose of the metal bushing, no? Lastly, there are the bearing bushings. CA mechboxes come with bearing bushings, and the small bearings that line the inside of the bushing allow the gear axles to cycle smoothly for a higher rate of fire.

TM Note:

If you are upgrading your spring, it is almost essential that you replace the nylon bushings in your gun with metal bushings. Nylon can’t take the excess friction involved with the replacement of an upgraded spring. Upgraded spring means more friction, friction means heat, heat makes nylon melt, get it?

IIl. The Shims:

Shims are probably the single most overlooked parts in upgrading mechboxes. They are small, metal washers that space the gears and bushings so that they are evenly and correctly spaced. Too many shims can easily cause lockup because of the sheer tightness of the gears against the bushings, and too few shims can cause stripped gears through too many points of contact. They come in various thicknesses, which allow the user to use fewer or more shims in a re-shim job so there aren’t too many points of contact between the shims themselves.

Re-shimming the mechbox is necessary if you are replacing gears, bushings, the mechbox shell, or any combination of these parts.

IIm. The Cut-Off Lever:

The cut-off lever is a small part on the underside of the mechbox that allows the semi-automatic mode to function. It engages the sector gear and trigger assembly to cut off the current from the battery as soon as a semi-auto shot is complete, forcing the user to re-engage the circuit to fire another shot.

The cut-off lever's position is marked.

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IIn. The Trigger Assembly:

Arguably the most complicated portion of the mechbox, the trigger assembly is the heart of electrical control inside the gun. There are several contact points that must engage for an electrical current to travel through the gun. The assembly includes the electrical contacts, numerous wires, the actual trigger, the safety lever, and a whole bunch of small springs.

During an upgrade, it is best to try and not mess with the trigger assembly, since it can be very confusing to put back together. In a TM V2/V3 upgrade, it is best to fiddle with the least amount of parts possible, so it’s relatively easy to put back together.

IIo. The Selector Plate:

Although it resides on the outside of a V2/V3 mechbox, the selector plate is the heart of controlling firing modes. It pushes on contacts on the inside of the mechbox, and depending on where it’s positioned, can also determine whether the cut-off lever works properly or not.

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When the user switches the firing mode from safe to semi to full, the selector plate is physically repositioned to achieve different functions. When set to semi, the selector plate positions the cut-off lever so that it will be engaged by the sector gear when a shot is complete. When the plate is set to full, the cut-off lever is in a place where it won’t be engaged, and thus allow the circuit to run unimpeded. When set to safe, the selector plate is removed from the contacts, and further engages a small lever that controls the safety lever in the trigger assembly, blocking the trigger from engaging the contacts.

V6 Note:

V6 mechboxes do not have a selector plate. Firing modes are switched by allowing the trigger different clearances when pulled. When set to auto, the trigger can be pulled far enough to hit a small lever that disengages the cut-off lever inside. When set to semi, the trigger cannot physically reach this lever, thus, the cut-off lever functions normally.

III. Open Sesame!

This is probably the single scariest part to a person new to mechboxes, because we’ve all heard of spring guides and cylinders flying across the room and the notion of parts flying out of the gearbox as soon as it opens. In all honesty, this can happen if you don’t keep the box secured in the right spots, so this section is to tell you what to watch for when opening your box.

First, you’ll need all of the materials necessary to crack the gearbox open. It is VERY helpful to have both a flathead and a Phillips head screwdriver, and a T10 driver if you’re working on a TM mechbox. After you have all of the tools required to open box, begin removing the screws around the gearbox, except for the one screw near the back of the gearbox and under where the spring guide resides. This will keep the box together while you begin securing parts.

Second, as you remove the screws around the mechbox, be sure to lay them out on a flat surface like a diagram, noting where all the screws are supposed to reside. If there were any washers on any of the screws, be sure you place them on the correct screw, and place them in your “screw map.” It will be essential to the box’s integrity to have the correct length screws for their respective holes.

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If you have a V3 or V6, you will need to get the stopper rail off the top of the gearbox. I have found that placing a flathead screwdriver on the end of the stopper rail and hammering the screwdriver gets it to slide off nice an easy. Don’t worry about using force; the stopper rail is VERY tightly jammed on the gearbox.

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Now is the time where an extra set of hands can be VERY helpful. If you don’t have another person to help you, be sure that you can secure both the cylinder and spring guide before you completely open the box. When you’re ready, remove the last screw, keeping the gearbox closed tightly, and place it in your screw map. Now secure the cylinder with your hand, and open the box VERY slightly, just enough to place a screwdriver in the spring guide’s hole. Then, very slowly, begin prying the gearbox apart, making sure to keep the cylinder and spring guide firmly secured. When you have enough room, let the spring guide out of the mechbox along with the spring, and remove the upper shell of the mechbox.

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Now you see the big jumble known as the “hell-hole” in front of you. Congratulations, you have successfully opened up your mechbox without hurting anyone in the general vicinity!

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For a large labeled version of this pic, [go to the original article and] click here.

IV. The Standard Spring + Bushings Upgrade for Dummies:

Now the reason you’re even in your mechbox in the first place is probably attributed to one of four things:

a) You’re upgrading your gun without those damned labor costs. b) You have to fix something or other. c) You’re trying to “get a feel” for the mechbox so when a problem arises, you can fix it later, or... d) You need to re-grease your mechbox for your (x)0,000 round tune-up.

Well, sorry to say for those with problem (b), but this article will not help you replace obscure parts that have broken. The best you can do is refer to the in-depth analysis of parts and try to diagnose problems yourself.

People with section (c), I will get to the closing of the mechbox later.

(a) and (d), you have come to the right place. This section is to help new upgraders get a feel for doing a standard spring + metal bushings upgrade, and to also help them re-grease their mechbox and shim the mechbox properly.

First off, you’ll need a few parts; the spring and bushings (duh), but also a new shim set and some white lithium grease. Because many of us have TM AEG’s (great starter guns, aren’t they?) and got bored with the relatively low stock power, the majority of you will need to remove the nylon bushings from the mechbox. Depending on how much your gun has been used (think shots fired); the bushings will be really easy to get out, or really hard to get out. Reason behind this is basic wear & tear. The bushings will almost mold to the mechbox's shape after heavy use, causing the bushing to warp around the outside of the mechbox shell. On a lightly used gun, the bushings will just sort of sit in their holes, with no obstruction to keep them from being removed.

Second, you’re going to want to remove the tappet plate and cylinder unit from the mechbox. On the underside of the tappet plate is a small spring that is connected to a nub on the mechbox shell. Very carefully, take this spring out with a thin screwdriver or something of that nature, and do not lose it! Set the tappet plate aside along with the nozzle, cylinder, and the tappet plate spring for later.

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Next, remove the bevel gear (the bottom one) and find the anti-reversal latch. There is a very small spring on this part as well, so make sure not to lose it. Just remove the latch from the mechbox for now, and set it aside with its spring.

To pop the bushings out of the top shell half, just give the bushings a good knock in the middle and they should pop out. Be very wary of extra shims, note where they are so you have a base to start upon later. These shims could be stuck on the inside of the bushings, or may still be on the gear axles when you remove the gears from the bushings. When you eventually remove the gears from the mechbox, you should make a “shim map” similar to the screw map you made for the shell of the mechbox, noting where the shims were placed on the gears. Once the gears have been removed, you will see the last three bushings on the bottom of the mechbox. Only one will be “pop-able” from the outside of the mechbox, since two are obstructed by the selector plate. If you can, try to remove those last bushings with some needle-nose pliers from the inside, but if you can’t get it out, don’t strain yourself, you’ll just have to be very careful in the next part.

Look at the outside of the left mechbox half at the selector plate. Notice the very small spring on the top side of it; you will need to remove that, preferably with a long, thin screwdriver. Though the process isn’t hard, be absolutely sure you do not lose this spring. Now, flip the mechbox half over so you once again see the inner workings of the trigger unit. In the very lower-right corner of the trigger mechanism, there is a small screw that is holding the safety lever down. Unscrew it from the mechbox, and remove the small spring holding the lever in. Then gently take off the safety lever from the other side. Now take a look to the left of the trigger; you should see the cut off lever, which is of silver color and rather oddly shaped. Remove the single screw that is in it, and then remove the lever. It is rather nicely fitted into the shell, so nudge it until it pops out. You can then slide the selector plate out of its “guide rails,” so to speak, from the outside of the mechbox. Now you can pop out the other bushings with no obstruction.

CA/ICS/AE Note:

Your guns already have metal bushings, and you don’t need new ones. Thus, you can completely bypass the removal of the selector plate and the re-shimming of your mechbox.

V6 Note:

There is no selector plate on your mechbox, so you do not need to worry about removing it.

Now you should have quite a number of parts lying on your workspace. Make sure you can differentiate between them, keep your “maps” intact, and get ready to replace the spring.

If you are working on a TM/ICS/AE piston head, you will have to be VERY careful in this next part. Take the needle-nose pliers and very gently begin to twist the piston head off, using the small rim in front of the o-ring as holds for the pliers. Once it is twisted off, remove the spring from inside the piston, and unlatch it from the piston head. Reinstall the piston head, again being VERY careful to maintain the round profile of the piston head. You do not by any means want a flattened side, you will lose compression, and in turn, FPS. Don’t worry about latching another spring on the piston head, aftermarket springs do not have that “hook” required to latch on to the piston head like the stock spring.

CA Note:

To the best of my knowledge, the springs in your mechboxes are not latched onto the piston head. You can simply just remove them and insert the new spring.

Now that the piston head is prepared, you can reinstall the selector plate. Place the plate in its guide rails again, and install the safety lever. Put in the side of the lever that is inside the mechbox and place the spring where it should go (engaging itself and metal cut off lever), then place the outer part of the lever in its hole, and screw the screw back in. Then simply nudge the cut off lever to the position it was originally in and screw its screw back in. Test to make sure everything is working properly by pushing the selector plate side to side, engaging the small safety lever in the process. On the inside, the long, black arm of the lever should block the trigger from engaging.

IVa. Shimming Guide

Now that the electric components are all in place, position the metal bushings into their holes and make sure they’re all the way in. Do the same for the other half of the mechbox shell, then place the gears in their respective bushings with the shimming configuration you had before you replaced the bushings. Do not put in any other parts except for the gears, their bushings, and everything in the trigger assembly. Then close the two halves of the mechbox together without screwing in any screws. Make sure the two halves are as closed together as you can make them, if they’re not, the shimming won’t be accurate.

Now, reach into the hole where the cylinder would be, and try to push the sector gear. If you can spin it easily, without any other weird grinding noises, you’re already done with step one. If you can’t spin the gears, or it’s very hard to spin the gears, something is way too tight, and the trial and error begins.

Open up the mechbox and remove all the shims on all the gears. Start with the spur gear, placing a small shim on its underside. Then place it in its position, and, while pressing on it, make sure it isn’t rubbing on the bottom of the mechbox shell. Close up the gearbox, and try using a long, thin precision flathead screwdriver and try spinning it. It should spin just fine, but you’re trying to find if it’s too loose. If you find it can slide between the bushings, open up the mechbox and add shims to the TOP of the gear, not the bottom. When you reach the point where the gear spins smoothly inside the mechbox, with no wobbling, you can move on. A good rule to follow is to tighten the gear to the point where a .1mm shim will cause it to become too tight.

Next is the sector gear. Try it with no shims first, and place it in its bushing. If the spur and sector gear touch each other, then you will need to install a shim on the underside of the gear. Keep doing this until the gears no longer touch, but the gap between them is VERY small. Now, do what you did for the spur gear, closing up the shell and looking for wobble in the sector gear. Also note that if you cannot get the mechbox shell to COMPLETELY close with no gap between the sides, you will need to start over with a smaller shim on the underside of the spur gear. After sufficiently tightening the sector gear (to the point of .1mm throwing off the balance), you may move on to the bevel gear.

Starting with no shims again, place the bevel gear in its bushing and close up the mechbox. The only way to know if the bevel gear is too loose is to know that the gears do not rotate very easily. Again, start tightening the gear with small shims, and every time you add a shim, check to see if you can rotate the gears with little to no effort. When you reach that point, use the .1mm rule, and when you find the correct configuration, you’re done.

To find out if any of the gears are too loose, try pushing a very thin screwdriver on the gear axles inside the bushings. If the axle pushes into the mechbox, then that particular gear is too loose. You do not want a gear that is wobbling inside, so tighten that gear up. To check to see if the sector gear is too loose, just reach inside and try moving it side to side, if it does, then that’s too loose.

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Yes, it is complicated, and it is time-consuming, but doing it correctly will prove to be immensely helpful in maintaining your inner mechbox’s reliability and durability. Your ultimate goal is to find a shim configuration that allows you to spin the gears freely and smoothly, but to keep the gear axles decently tight inside the mechbox, so that you are unable or VERY, VERY slightly able to push them in and out of the bushings.

IVb. Greasing the Mechbox

Ok, now you’re done with shimming, and it’s time to make sure those new bushings won’t produce much friction. Take the white lithium grease and place a small dab of it on the inside of the 6 bushings around your mechbox. Next, wipe off any grease that has turned black on the gears or piston. Then, without losing your shim configuration, place small amounts of grease on the teeth and axles of the spur and bevel gear, so they engage each other and the bushings more smoothly. Take the piston’s rails on its sides, and place a marginal amount of grease inside those rails. Place some more grease on the piston's teeth as well. You may also please to put a marginal amount of grease on the coils of the new spring, so when it cools off, it doesn’t tear away the inside of the piston. Finally, very lightly apply grease to the "rails" on the underside of the tappet plate, this will prevent the piece from scratching against another part.

If you so please, you can also grease the guide rails on the inside of the mechbox where the piston slides back and forth, and also on the outside of the cylinder head’s nozzle, but do not go crazy with the grease. Like alcohol, grease in the mechbox should be used in moderation. Using too much grease will cause more parts to stick together, notably the piston head o-ring. Also, do not use it on parts that obviously don’t need it! This includes the trigger assembly, and spring guide, the selector plate; they do not need the grease! Remember, any dirt, grime, sand, and other foreign material that finds its way into the mechbox will be attracted by the grease, so you want to have just enough to smooth movement between the parts, but not so much that it acts as a debris magnet.

V. Getting the Mess Back Together

Well, you know what the stuff in your mechbox does, and it may even be newly upgraded. However, a mechbox with a stiffer spring won't function if it's laying in pieces on the ground. So, here's how to close up the mechbox without breaking anything in the process.

Now, it’s time to start re-installing parts to where they should go. First and foremost, place the anti-reversal latch where it resides, in the small hole to the left of the bevel gear bushing. Then place the spur gear in place, and get the bevel gear notched into its bushing. Pull the anti-reversal latch so that it engages the bottom side of the bevel gear. If it is on the other side, it will literally stop your mechbox cold and possibly strip a tooth or two.

Before you get the cylinder unit in, test the compression that it holds with the piston head’s o-ring. Place the nozzle on the cylinder head and plug it up with your finger. Now take the piston and try to shove it down the cylinder. If the compression is perfect, the piston head should be almost impossible to move down the cylinder. If there is a small leak somewhere in the system, localize its source and deal with it accordingly. Usually, problems with compression are because the o-ring is not engaging the cylinder wall, so check to make sure it isn’t flooded with grease or too small. When you get the compression working nicely, move onto re-installing all the parts.

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When all the gears are in place, put the entire cylinder unit and all its subsidiaries in the mechbox. Make sure the small holes in the cylinder head fit into the small metal nubs on the inside of the mechbox, and that the tappet plate goes in the small space where the cylinder head isn’t. Then take the small tappet plate spring, hook it onto the tappet plate’s “hook”, and re-engage it onto the small nub where it once resided. Now take a look at the tappet plate’s arm; it should be on the other side of the sector gear, and the small nub on the inside of the sector gear should be in position to engage the arm on the tappet plate.

Now, you can take the bushings that go onto the top sides of the gear axles and install them onto the gear axles. Again, take note that the tappet plate is engaging that nub and is under the sector gear bushing. Retrieve the piston with head and the spring installed in it, and position the piston inside the cylinder. Again, an extra set of hands will be helpful, since this part is also rather tense.

When the piston is all the way in the mechbox, check all the other parts first to make sure nothing will make you take it all apart again. Check the following things:

• Anti-reversal latch: Is it engaging the bevel gear and in its slot as dictated?
• Tappet plate: Is the spring installed, is the nozzle fitted into the plate’s slot? Is the plate's "arm" under the sector gear bushing, engaging the nub on the gear?
• Cylinder: Is the hole in the cylinder in the correct place?
• Cylinder head: Are the holes lined up with the nubs on the mechbox wall?
• Trigger Assembly: Does the trigger assembly work? Does the safety engage? Do the contacts on the trigger and the “wiring box” engage?
• Piston: Is it in so the teeth are facing down? Are the guide rails in the correct place?
• Spring Guide: If there are tabs on your spring guide, is one pointing straight up out of the mechbox?

If you are sure that all of these things are done and accounted for, you can then jam the spring and spring guide in the mechbox, and place the spring guide in its little notch in the back of the mechbox. Now, take the other mechbox half and place it on top of all of it, fitting the cylinder into its hole, and getting the spring guide to stay put. Don’t worry about the gears and bushings in their holes just yet.

If everything on the top half is secure, i.e. the cylinder is in the correct spot, the cylinder head is fitted into the shell, and the two shell halves are closed tight, you can screw in a couple screws in various positions along the mechbox. Now looking at the outside, find the bushings and get them into place. You can tap the mechbox wall with a blunt, plastic object to kind of oodle them into place, and then push down the mechbox as the bushings fall into their respective holes. To get the anti-reversal latch, stick in a very thin screwdriver through the crack in the mechbox wall and position the anti reversal latch. When you are certain everything is in place, try to get the two halves of the mechbox together. You might hear a slight click noise, but that’s just everything falling into place.

When you are certain everything has fit together, screw in all the screws around the mechbox according to your screw map, and do a couple things to make sure things are functioning properly:

• Air Nozzle: Push in on it. Does it go in and then spring back out of the mechbox? It should.
• Trigger: Does it do what it’s supposed to do?
• Selector Plate and Safety Lever: Push the selector plate all the way so it engages the safety lever. Is the trigger blocked? Does the selector plate’s spring perform its function?

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If everything is fine, then re-install just the motor, whether it is by a motor cage, or a pistol grip. When all the wires are connected, try to fire the gun. Does the air nozzle cycle in and out of the mechbox? When firing full-auto, does the piston sometimes lock back after a decent burst? If it does both these things, fire once on semi (get a feel for where the selector plate should go for this function) to reset the spring and gears, and reinstall the whole deal back into your gun.

If something is wrong, go back into your gun and find the problem. Usually, gear lockup is a pretty obvious thing to diagnose. But if your shimming job was correct, check for stripped teeth on any of the gears, including the pinion gear on the motor. The problem has sometimes been attributed to a badly adjusted motor because of the new shimming, but this is not usually the case. Also check to see if your battery is fully charged (you have no idea how many people have had to break everything open, only to find out their battery wasn’t charged), and try again. You should be able to get everything cycling smoothly after (at most) two or three tries.

If you just can't get the gun to shoot properly, don't fire it excessively to see if it "breaks in", because the gearbox will do just that, break.

Be sure to chronograph your newly upgraded gun, to make sure everything is performing nicely.

Congratulations! You have upgraded your gun!

VI. Maintenance and Closing

So you’ve successfully upgraded your gun, you’ve learned how your mechbox works, among other things, but it’s not over yet. In order to make sure you get the most out of your gun, you’ll need to do a few things every once in a while to make sure it runs smoothly ‘til high heaven come runnin’.

IVa. Motor Adjustment

If you have reshimmed your mechbox while upgrading it, it will probably be necessary to adjust the motor height to the proper position of the newly shimmed bevel gear. Depending on what gun you have, there either be a small 1.5mm allen screw on the bottom of your pistol grip, a flathead screw on the bottom of your pistol grip, or an adjustment screw on the actual motor (as is the case with the P90).

When you find that screw, fire the gun on semi-automatic mode, and note the pitch of the noise that emanates from the mechbox. Loosen the screw half a turn at a time. If the sound becomes higher-pitched, stop, and begin tightening the screw. Again, only tighten it a half-turn at a time. When your gun as a whole is at its quietest, stop turning the screw, and secure the screw it to its correct position using locktite or similar adhesive. This will make sure your motor stays at its correct position and not come ajar after long bouts of firing.

VIb. Maintaining the Inner Barrel and Hop-Up Assembly

Since you have been to your mechbox and back out again, there’s no telling what sorts of weird gunk got into your hop up unit while it was out of the gun. To clean the entire assembly, be sure the hop-up is at its lowest setting, and do these things...

Take the long, thin cleaning rod that was supplied with your gun, and thread a small piece of cotton cloth through the “needle-eye” end of the rod. Spray this cloth with 100% silicone oil and thoroughly swab the inner barrel. You should keep swabbing until the patches come out clean, with no black spots or otherwise.

Next, drop a few drops of the oil into the actual feed tube of the gun. Shake it around a bit so it gets to the hop-up bucking.

Finally, when you’re ready, shoot the gun about 100 times (with BB’s) with the hop-up on to get the silicone oil to settle. The gun may fire very awry about 10-20 times; this is because any excess silicone oil in the barrel will affect the BB’s path as it travels down the barrel. When you notice the hop-up returning to its normal function, you can stop firing BB’s, as the silicone oil is now fully spread and won’t cause any adverse effects. Depending on the environment you play in, you may have to perform this entire process every 1000-5000 rounds, or when you notice excessive jamming, double-shots, or adverse accuracy.

VIc. Closing Comments

Well, you’re all done now, so there’s just a few things I should leave you with before I finish off this article.

First, if you plan on upgrading your gun further than, say, an M100 (PDI 130) spring, you will probably need to replace more than just the bushings and shims. When you go into the 400-450 FPS range, you will need to buy more reinforced parts, since just popping in the new spring will cause numerous problems. Be logical. Don’t just think your mechbox and its parts can handle it, because chances are, it can’t. When you upgrade into that high range, be thinking about what parts will take all the stress, and replace those parts when you upgrade your spring. Main rule: don’t be stupid, don’t get greedy. Take some time to make sure your gun won’t blow up on you. Don’t think, “Oh, an extra 30 FPS won’t hurt...” because, I’m sorry to say, but it will. Buy the extra parts, don’t go bare minimum, as you won’t be too happy in the future.

Second, when you do upgrade your gun, don’t automatically think you’ll need a bigger battery to power it! Chances are, this is NOT the case. When my friend was looking at ways to reduce his ROF so his magazines could keep up, he powered an M120 equipped TM G36 with a 7.2v battery. His ROF? About 10-11 RPS. If you buy a super-duper 10.8v battery on an M120, your motor and gears will NOT be too happy with you. If you absolutely must have that 2 RPS back from upgrading, just remember, your gun is shooting at a much higher velocity now, and you should be hitting targets further away without the aid of the well-known philosophy “spray n’ pray.”

Third, remember this sentence: One of the greatest common myths and fallacies of airsoft weapons is “FPS makes the gun.” Many experience upgraders and myself have only one response, “No, it doesn’t.” Why? Because what good is FPS when your gun is shooting at a 15 FPS difference per shot? What good is FPS when your gun strips all three gears? What good is FPS if your fuse blows up? What good is FPS when your mechbox shell cracks in half? are some of the few questions to think about when upgrading an AEG. Lower FPS upgrades like 330 FPS won’t require anything much else except the metal bushings, but a few other parts can make your gun much more consistent and fun to use as a whole. Think about replacing some of the compression parts in your mechbox, like the piston head and the air nozzle. Think about replacing inner barrel and hop-up components to achieve maximum accuracy. And most of all, think about what you’ll do when your gears strip. Being prepared and having a good, consistent mechbox will give you hours of shooting fun without the hassle of having to repair your mechbox over and over again.

Last but not least, go kick some ass! You’ve got the internals and the know-how to do it, so go for it!

I'd also like to take some time to thank several people who helped me with this article. First, ThaFlash, for clarifying several points in the shimming guide and upgrade section. Second, Hitlery, for supplying the gearbox used is most of the pictures. Third, Xplosive, for helping me with all the dirty work involved in that particular mechbox. And finally, socomairsoft, for all his help with editing and HTML-ing (new writer's verb!) this article.

Addendum

[in the original article] Below is a gif image of how all the bits in your gearbox work together. ASR thanks Bolduerbum for creating this image.

http://www.airsoftretreat.com/gallery/data/573/vidfingif.gif