This article will discuss the standard upgrades done on AEGs to increase muzzle velocity and accuracy from a broad perspective. While I won’t go into great detail into which exact pieces and part numbers you may want to look at for your exact AEG, I will approach a certain area of products with a general idea of what that part does and why you might want to upgrade it and occasionally give a few popular choices for an area.
For starters, this is your gearbox if you have a version 2 gearbox design. For the most part, this diagram covers version 3 as well. That covers any M4/M16 variant (aside from those made by ICS), AK, MP5, G3, G36, and I might have missed more. Most other gearbox derivatives (with the new Systema Revolution and AEP designs excluded) will also have the same components, if just in slightly positions.
Metal Bushings/Bearings (adds to durability):
What the hell is a bushing? Bushings are what keep your gears in place so they don’t wobble around as they spin. Gear wobble can lead to locking up and/or breaking of the gears, piston, or tappet plate. Bushings are basically just a fancy washer that fits in a hole. A bearing serves the same function, but at with the benefit of less friction between the gear axis and the bearing/gearbox (and at the expense of being a little less durable - but they’re far better than the nylon bushings). Less friction will save you a bit of battery life, since the motor won't have to overcome extra friction to turn the gears at the same rate. The difference is pretty minor, and both are considered good upgrades.
The gear has a peg in the center of it that fits inside the bushing/bearing, which fits inside the gearbox shell. If everything fits properly (and the gears are correctly shimmed, which I’ll get to later), that won’t wobble. If the gears wobble, the teeth may (read: will) eventually break teeth off from pressure being applied to an area at an angle that the gear isn’t designed to handle. It’s important that you protect your gears from damage, as a complete set of three gears will be one of two most expensive internal pieces of your AEG.
If your AEG was made by Tokyo Marui or a Chinese clone of TM, there’s a good chance that you have nylon bushings in your gearbox. Nylon, for all intensive purposes, is just a fancy word for plastic. Nylon bushings warp, bend, and if it gets really cold, can get brittle and break. Broken or bent bushings will lead to catastrophic gearbox failure.
Since a set of metal bushings will only cost $15 from almost every airsoft retailer out there (bearings being another $10-$20 more), it’s recommended that this be the very first upgrade you do, as it will keep your gears from breaking as you continue to upgrade your AEG. If you’re buying bearings, be sure to get the right size (they range from 6mm to 8mm). 6mm bearings are generally discouraged because they are more fragile than the bigger sizes, so if you have a gearbox shell with 6mm holes, I’d suggest going with bushings unless you want to replace your gearbox shell for the ability to use bigger bearings.
Shimming:
Andrew put a pretty basic guide to shimming up a few days ago, so I'll edit that in since it was brought to my attention that there was never a reference back to shimming. There are several ways of shimming, but I think this is probably the most common and the easiest.
Panzerfaust wrote:You can check shimming by opening the gearbox, removing the cylinder and spring assembly. Close the gearbox with enough screws to get everything tight. Putting your finger into the space left by the removed cylinder a$$'y try to spin the sector gear (gear that pulls piston). If you can spin your entire gearset easily, check the gearshafts for side to side movement. You can do this by poking the shafts with a screwdriver or something and seeing if you observe any give, or by shaking the gearbox and listening for any rattling. If there's noticeable side to side movement, isolate it to the gear, open the gearbox and add a shim. Close it all up and repeat as necessary until you are able to spin the gears easily with little-NO side to side.
Spring (significantly affects muzzle velocity):
At $8 - $25, your spring is one of the least expensive pieces you’ll buy for your AEG. It’s also the one part that will contribute to the final muzzle velocity of your AEG. On the other hand, too strong of a spring can actually crack the front of your gearbox clean off. It is generally accepted that an M120 (on a PDI spring, this equates to a 170%) spring will put you at or around 400FPS, which is the standard velocity limit for outdoor use in AP events. Upgrades past an M120 will probably put you over this limit, and might crack (or completely snap off) the front of your gearbox as well.
Springs come in two different styles: regular and irregular pitch. The difference between the two is the way the spring is coiled. Irregular pitch springs have a tighter coiled area of spring in the center so that when the first few teeth of the piston grab onto the spinning gears, the full tension of that spring isn’t applied to only a few teeth, which might be enough to break them off. On the other hand, many poly-carbonate pistons have the first tooth made of steel or other reinforced material to prevent this from happening on a regular pitch spring, so there are varying schools of thought as to which is actually “better.â€
