AEG Upgrades FAQ

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AEG Upgrades FAQ

Inside the AEG gearbox: gears, motors, springs, pistons, air seal, and electronics — what each upgrade changes and when it is worth doing.

Why does matching cylinder volume to barrel length matter in an AEG?

Matching cylinder volume to barrel length keeps the air supply filling the barrel for the whole BB travel without wasting air or running short. A cylinder that is too small for a long barrel can lose pressure before the BB exits, hurting consistency, while excess volume on a short barrel wastes air and can increase noise and muzzle blast. Builders compare the cylinder's effective air volume, accounting for ports, to the barrel's internal volume, aiming for a ratio suited to BB weight and spring strength.

What is the difference between a ported and a non-ported cylinder?

A ported cylinder has a hole in its side that lets air escape until the piston passes it, which reduces effective volume for shorter barrels. A non-ported, or full, cylinder uses its entire volume and suits longer barrels. Choosing between them is about matching volume to barrel length, not about one being an upgrade by itself. Using a full cylinder with a very short barrel mainly wastes air, while a ported cylinder on a long barrel can starve the shot and reduce consistency.

What is the purpose of shimming an AEG gearbox?

Shimming sets the side-to-side position of each gear on its axle using thin metal washers. The goal is minimal free movement without binding, so gears mesh cleanly, spin freely, and sit at the right height relative to each other and the cutoff and trigger contacts. Poor shimming causes whining, grinding, extra motor load, and faster wear. Good shimming is checked by feel and sound with the gearbox closed, and it is a patience task rather than a parts purchase.

How do you adjust motor height on an AEG, and what happens if it is wrong?

Motor height adjustment sets how deeply the motor pinion gear engages the bevel gear inside the grip. A motor set too low engages only the edge of the gear, causing grinding, heat, and rapid wear, while one set too high can bind the drivetrain and strain the battery. Most grips have a screw in the base plate for this adjustment. Turn it in small steps, listening for the quietest, smoothest sound on semi-auto, then confirm the motor is not being forced against its endbell.

What benefit does an air nozzle with an internal O-ring provide?

An air nozzle with an internal O-ring seals better against the cylinder head tube than a plain nozzle, reducing air that leaks sideways during each shot. That tighter seal usually improves shot-to-shot consistency and can recover a small amount of lost velocity, because more of the cylinder's air goes behind the BB instead of escaping. It is not a power part by itself, and fit matters: the nozzle must still move freely, align with the hop-up, and match the model's specified length.

What does a delay chip on the sector gear do in an AEG?

A delay chip, or delayer, is a small clip on the sector gear that holds the tappet plate back slightly longer, giving each BB more time to feed into the chamber. It mainly helps high-rate-of-fire builds and some magazine combinations that otherwise skip feeding. On a stock gun that already feeds reliably, a delayer adds nothing, so add one only to cure a demonstrated feeding problem at your actual build speed.

What is the difference between bushings and bearings in an AEG gearbox?

Bushings are solid metal rings that the gears rotate in, prized for strength and simplicity, while bearings use tiny ball bearings for lower friction and suit higher-speed builds. Bearings can fail under heavy spring loads if undersized, so many durability builds use bushings, especially under the sector gear. Either choice must be seated flush and matched to the shell's axle hole size, commonly 8mm or 9mm.

How does a ported piston head change an AEG's performance?

A ported piston head has small holes in its face so that, as the piston accelerates forward, air pressure expands the O-ring outward against the cylinder wall for a better seal. This improves compression consistency compared with a loose plain head. It does not add power by itself; it recovers power lost to leakage. Pair it with a correctly sized O-ring and cylinder grease, and verify results over a chronograph.

What does an electronic trigger unit add beyond a basic MOSFET?

An electronic trigger unit replaces mechanical trigger contacts with sensors and a control board, adding programmable fire modes, burst settings, trigger sensitivity, precocking, and diagnostics such as battery and gearbox monitoring. A basic MOSFET only protects contacts and improves trigger response. ETUs reward players who use the features, but they add cost and setup complexity, and wiring or sensor faults need more methodical troubleshooting than a simple contact system.

Why do builders radius a gearbox shell's front corners?

Radiusing means filing a small rounded relief at the front corners of the cylinder window, where impact stress concentrates each cycle. The rounded profile spreads stress and reduces the risk of the shell cracking at those corners, a known failure on high-stress builds. It is a careful, irreversible modification done with the gearbox empty and clean, and it protects longevity rather than adding any velocity or rate-of-fire performance.

What is a full-steel-teeth piston, and when is it worth choosing?

A full-steel-teeth piston replaces the plastic rack teeth with a steel rack so the teeth survive high-stress springs and high-speed builds where plastic teeth strip. It is worth choosing when your build's spring strength or rate of fire has already damaged a piston, or as preventive durability in a demanding setup. For mild stock builds it adds cost and a little weight without benefit, since the original rack is not the weak point there.

What does a sorbothane pad on an AEG cylinder head actually do during each shot cycle?

A sorbothane pad cushions the piston's impact on the cylinder head at the end of each shot cycle, spreading the load and reducing the sharp shock sent into the front of the gearbox shell. That softer impact quiets the mechanical slap and can help the shell last longer in higher-stress builds. The pad's thickness also slightly changes piston position, which is why builders check angle of engagement after fitting one. Use the correct hardness and thickness for your spring and rate of fire, not the thickest pad available.

How do high-torque and high-speed AEG motors differ, and which builds does each suit best?

High-torque motors turn stronger gear sets and heavier springs with less strain, giving crisp semi-auto response, while high-speed motors spin faster and raise full-auto rate of fire on lighter setups. Choose high-torque for stronger springs, lower-ratio gears, and responsive trigger builds, and high-speed only for mild springs and stock-ratio gears where current draw stays manageable. Pairing a high-speed motor with a heavy spring causes heat, overspin, and premature wear. In most modern responsive builds, a balanced high-torque motor is the safer default.

What changes in trigger response and rate of fire when you switch an AEG from 18:1 to 13:1 gears?

Switching from 18:1 to 13:1 gears makes the motor complete each shot cycle with fewer rotations, so semi-auto response feels snappier and full-auto rate of fire rises. The tradeoff is higher load per cycle, more current draw, and greater risk of overspin or premature engagement on lighter setups. The change does not by itself raise velocity, which is set mainly by the spring and air seal. Pair lower-ratio gears with a suitable high-torque motor, correct shimming, and a battery that can supply the extra current.

How do you test AEG air seal by checking piston and cylinder compression by hand?

Test compression with the gearbox open or the cylinder assembly removed: cover the cylinder head nozzle with a fingertip, then push the piston forward by hand. A good seal resists firmly with smooth, even pressure and no hiss. If the piston slides easily or air escapes in bursts, inspect the piston head O-ring, cylinder head seal, and cylinder for scoring or a poor fit. Repeat the test with the air nozzle fitted and its tip covered to include the nozzle seal. Fix leaks before changing springs.

What does a bearing spring guide change inside an AEG gearbox?

A bearing spring guide lets the mainspring rotate slightly as it compresses and releases, instead of twisting against a fixed guide. That reduces spring torsion, helps the spring return more consistently, and can slightly smooth the cycle and reduce wear on the spring and piston. Bearings in the guide also add a small amount of preload, so velocity may rise marginally. It is a supporting part rather than a power upgrade, most useful in higher-rate or stronger-spring builds where spring twist and fatigue become noticeable.

What does a brushless AEG motor change compared with a brushed motor?

A brushless motor removes the brushes and commutator that wear and spark in a brushed motor, using electronic control to spin the rotor instead. In an AEG that means stronger and more consistent torque, less heat for the same work, and no brush replacements over its life. Trigger response usually improves, especially with a matched electronic trigger unit. The trade-offs are higher price and dependence on electronics, so confirm compatibility with your trigger unit before buying.

How do capacity in mAh and discharge rating in C affect an AEG battery upgrade differently?

Capacity in mAh sets how long the battery lasts, while the C rating, multiplied by capacity, sets how much current it can deliver for trigger response and rate of fire. A larger pack that cannot deliver current will still feel sluggish; a high-discharge pack that is small will feel sharp but run flat sooner. Fit also matters, because the pack must sit in the stock or handguard without pinched wires. Upgrade both figures within the space your gun allows.

What does a steel anti-reversal latch do in a high-stress AEG build, and when is the stock latch a weak point?

A steel anti-reversal latch holds the bevel gear against spring pressure whenever the motor stops, preventing the gearbox from spinning backwards between shots. The latch itself is small, but its tip and pivot take repeated impact, especially with strong springs and high-torque motors. A soft stock latch can round over or bend, after which the gun may click backwards, lose precocking position, or stress the piston. A hardened steel replacement is a durability upgrade rather than a performance gain: it does not raise rate of fire, but it protects the rest of a high-stress build.

What does low-resistance wiring actually improve in an AEG, and what does it not change?

Low-resistance wiring improves how efficiently battery power reaches the motor by reducing voltage lost as heat in thin, high-resistance stock wire. Thicker, better-conducting wire and clean soldered joints can slightly sharpen trigger response and reduce warm wiring on demanding builds. It does not add power by itself, change gear ratio, or fix a weak battery or motor. The gains are modest on a mild stock gun and larger on high-current setups. Keep wire routing tidy and insulated, because a pinched low-resistance loom that shorts is worse than the original wiring.

When should you replace a tappet plate, and how does its timing affect nozzle movement and feeding?

Replace a tappet plate when its fin is worn, cracked, or bent, or when the air nozzle no longer moves back and forward fully and evenly. The sector gear pulls the plate and nozzle rearward to let a BB enter the chamber, then the plate spring snaps the nozzle forward to seal against the bucking. A tired plate causes misfeeds, double feeds, or a poor air seal that looks like low power. On fast builds, a shortened or delayed plate timing can improve feeding, but material strength matters more than aggressive trimming.

What is a bore-up cylinder kit, and why does it only help certain barrel setups?

A bore-up kit increases an AEG's swept air volume by using a slightly wider cylinder, matched piston head, cylinder head, and nozzle. It helps only when the barrel setup actually needs more air than a standard cylinder delivers, typically long or wide-bore barrels that leave the BB still accelerating as the piston finishes. On a short barrel, extra volume is wasted and can add noise and stress without raising consistency. Bore-up parts must be bought as a matched set, because standard-diameter heads will not seal correctly inside the wider cylinder.

How does fitting a lighter handguard change an AEG's handling without changing its internal performance?

A lighter handguard improves handling by removing weight from the front of the rifle, where it affects balance and fatigue most, without changing internal performance at all. The gearbox, barrel, and hop-up determine power and accuracy; the handguard is a mounting and handling component. A lighter rail makes target transitions faster and long games less tiring, and modern designs still provide accessory mounting for grips, lights, and sling points. Check barrel nut compatibility and that the new handguard does not flex against the outer barrel, which could disturb consistency if it makes contact under load.

What does a reinforced tappet plate do in an AEG?

A reinforced tappet plate does the same job as a standard one, moving the air nozzle back and forth in time with the gears so BBs feed and seal correctly, but it does it with greater resistance to flexing and breakage. The tappet plate is a thin part under repeated spring tension, and its front section is a common failure point in hard-running builds. A reinforced version, often in tougher polymer with extra material at stress points, suits higher-stress setups. Correct timing and fitting matter more than strength alone.

How do helical-cut AEG gears differ from straight-cut gears in noise and shimming?

Helical-cut gears mesh with angled teeth that engage gradually, so they usually run quieter and spread load across more tooth surface than straight-cut gears. The trade-off is side thrust: the angled mesh pushes gears sideways along their axles, which makes careful shimming and solid bushings or bearings more important. Straight-cut gears are simpler to shim, tolerate looser setups, and produce a sharper mechanical whine. Choose helical sets for a refined, quieter build you are willing to shim precisely, and straight-cut sets for straightforward durability and easier setup.

Why do builders check piston weight when choosing between high-speed and high-torque AEG builds?

Builders check piston weight because it changes how fast the drivetrain cycles and how hard the piston arrives at the front of the cylinder. A lighter piston suits high-speed builds, since it accelerates and stops with less momentum, reducing stress at very fast cycle rates. A heavier piston can help a high-torque build driving a stronger spring, adding momentum that supports consistent compression, but it also increases impact energy and can slow the cycle. Matching piston weight to gear ratio, spring strength, and BB weight keeps the build efficient instead of simply chasing one headline number.

What does a reinforced gearbox shell change besides strength, including weight and fit?

Besides strength, a reinforced shell usually adds weight and changes fit, because its thicker walls and extra material take up more room inside the receiver. Some builds need light sanding, different bushings, or re-shimming before everything closes without binding. The tighter, stiffer casting can also hold gear and bearing alignment more consistently, so meshing stays stable over time. Before buying, check that the shell matches your receiver, motor cage style, and wiring route. A stronger shell that forces parts out of alignment can create more problems than the standard shell it replaces.

If a build already has a bearing spring guide, does adding a bearing piston head still help?

Adding a bearing piston head can still help, because the two bearings do the same job at opposite ends of the spring: both let the spring rotate as it compresses and extends instead of winding up with stored twist. In practice one bearing, usually at the spring guide, removes most of the twisting, and the second adds a smaller improvement plus a little piston weight. That extra weight slightly slows a very high-speed build, so speed-focused players often run the guide bearing only. For a standard or torque-focused build, running both is harmless and keeps spring behaviour at its most consistent.

How does cylinder head pad thickness change piston rest position when you correct angle of engagement?

Cylinder head pad thickness changes piston rest position because a thicker pad pushes the piston slightly further back at rest, which rotates where the sector gear first meets the piston's tooth rack. That small shift is exactly what angle-of-engagement correction uses: the first tooth contact moves onto the stronger part of the pick-up tooth instead of its fragile tip. Adding pad thickness without checking can go too far and cause its own clash, so builders test engagement by hand through the shell window or with marking dye. The pad also absorbs impact noise, but its spacing effect is the reason thickness must be chosen deliberately.

Why must cylinder port position be re-checked after changing to a much longer or shorter inner barrel?

Cylinder port position must be re-checked after a barrel change because the port decides how much air the piston delivers, and that volume should roughly match the volume of the barrel it feeds. A much longer barrel on a heavily ported cylinder runs out of air before the BB leaves, wasting energy and hurting consistency, while a short barrel on a full cylinder vents excess air as extra muzzle blast. Matching volume keeps velocity efficient and groups stable. Builders either fit a cylinder ported for the new barrel length or confirm by chronograph and grouping tests that the existing pairing still behaves.

Why should you fix air-seal losses before fitting a stronger spring in an AEG?

Fixing air-seal losses comes first because a stronger spring cannot recover velocity that is leaking past the piston, nozzle, or hop-up. Seal work on the piston O-ring, cylinder head, nozzle, and bucking often restores lost performance with no extra stress on the gearbox, motor, or battery. Adding spring power to a leaky build raises wear and current draw while shot consistency, the thing players actually notice, stays poor. Once compression tests show a stable seal, measure again: many builds no longer need the stronger spring, and any that do will use it far more efficiently.

What does lightening an AEG piston change, and what can go wrong if you remove too much material?

Lightening a piston reduces moving mass, so the gearbox cycles faster and returns quicker in high-speed or responsive semi-auto builds. Removing material from the piston body, often called swiss-cheesing, lowers the energy wasted accelerating the piston itself and can reduce premature engagement stress at high rates. The risk is structural: drill too close to the rack, head, or spring seat and the piston flexes or cracks under load, shedding debris into the gearbox. Keep cuts symmetrical, leave generous material around load paths, deburr every edge, and treat the part as a wear item afterwards.

How does shimming a hop-up arm to remove side play improve shot consistency?

Shimming a hop-up arm removes side-to-side play so the nub presses the bucking straight and identically on every shot. If the arm can wander, the contact patch shifts slightly between shots, applying a little more pressure on one side and sending BBs hooking left or right without any change to the dial. Thin shims on the arm's pivot pin, fitted so the arm still moves freely up and down, centre that pressure. Test with a long string of shots at a fixed target: the group should tighten horizontally before you change anything else.

Why should you make AEG upgrades one stage at a time and re-test between stages?

Making one change at a time lets you prove each upgrade helped and catch a bad part or fitting error before it is buried under more changes. Fit one stage, then chrono a full shot string, check feeding on semi and full-auto, and shoot groups at a fixed distance, recording the results. If performance worsens, the last change is the prime suspect and can be reversed cheaply. Stacking a spring, gears, motor, and barrel in one session may work, but when it does not, diagnosis takes far longer than the staged testing ever would.

Why does a replacement AEG air nozzle that is slightly too long or too short cause feeding or air-seal problems?

Even a fraction of a millimetre of nozzle length changes both feeding and sealing, because the nozzle must meet the bucking lips at exactly the right moment. A nozzle that is too long can push into the bucking, deform its lips, and slow or block BBs entering the chamber. One that is too short leaves a gap that leaks air on every shot, dropping velocity and consistency. Always compare a replacement against the original nozzle, measure overall length, and test feeding and chronograph results before assuming a spring or bucking fault.

What is the difference between a full-tooth and a half-tooth AEG piston, and why must it match the sector gear?

A full-tooth piston has tall teeth along its rack, while a half-tooth piston has shorter teeth that leave clearance for thicker sector gear teeth. High-ratio and dual-sector style gears often need that extra clearance, so they pair with half-tooth pistons. Fitting a full-tooth piston where clearance is tight can make the sector gear strike the rack incorrectly, jamming the cycle or stripping teeth. Fitting a half-tooth piston in a standard build can reduce engagement strength. Match piston tooth form, gear set, and spring load as one system, not as separate parts.

What do spring preload spacers do in an AEG, and why do they change velocity without changing the spring?

Preload spacers raise velocity by compressing the spring a little more at rest, so it pushes harder through the whole stroke. Spacers sit on the spring guide or piston and effectively shorten the space the spring occupies. A small spacer gives a modest gain, which suits fine adjustment near a field limit. Too much preload makes the spring coil-bind, strains the motor and piston, and can prevent the gears completing a cycle. Add spacers one step at a time, chronograph after each change, and stop well before the action feels laboured or strained.

What is premature engagement in a high-speed AEG build, and what causes it?

Premature engagement happens when the sector gear picks up the piston before the piston has fully travelled forward, so the gear chews the rear rack teeth instead of starting a clean cycle. It appears when three factors combine: a heavy piston, a weak spring that pushes it forward slowly, and a high rate of fire from fast gears or an 11.1-volt battery. Warning signs are a grinding burst, sudden velocity loss, and plastic or metal shavings from damaged first teeth. Fixes follow a set order: lighten the piston, fit a stronger spring, or reduce rate of fire, changing one factor at a time and inspecting the rack after each test.

Why do builders glue the steel rack into a polymer AEG piston?

Builders glue the steel rack because on many polymer pistons it is only press-fitted or lightly moulded in, and repeated high-stress cycles can work it loose in its slot. A loose rack shifts under load, changes how the sector gear meets the first tooth, and can tear free completely in a fast or strong-spring build. The fix is simple: remove the rack, degrease rack and slot, apply a thin film of two-part epoxy along the slot, reseat the rack, wipe away squeeze-out, and let it cure fully before reassembly. Afterwards check the rack sits flush and level, because a crooked rack causes the same pickup problems as a loose one.

What is mid-cap syndrome, and why does a brand-new mid-cap magazine cause misfeeds?

Mid-cap syndrome is misfeeding and velocity loss caused by a brand-new mid-cap magazine whose spring is still at full strength. The stiff spring pushes the BB stack up so hard that the top BB presses on the air nozzle, adding friction, slowing the nozzle, and sometimes costing velocity or causing skipped shots. It fades as the spring settles. Break a new magazine in gradually: load it to about three-quarters capacity for the first few games, cycle several full loads through it at home, and avoid leaving it fully compressed for weeks. If one new magazine causes problems older ones do not, the magazine, not the gun, is the first suspect.

What does cycle detection on an electronic trigger unit actually sense?

Cycle detection means the trigger unit watches the sector gear itself, usually via a small cam and sensor or a magnet, so it knows when one full gear rotation has completed. On semi-auto that lets the unit cut power at the same point every shot, giving a complete, consistent cycle instead of stopping whenever a mechanical lever happens to trip. It also enables features like pre-cocking and reliable burst counts, because the unit counts real cycles rather than estimating time. When fitting one, alignment is critical: if the sensor sits slightly off the cam, the unit miscounts, causing double shots on semi or cycles that stop half-open.

Which teeth should you remove when short-stroking a sector gear, and what does each tooth cost?

Remove teeth from the release side of the sector gear, meaning the last teeth that engage the piston, and never from the pickup side. Taking teeth off the pickup side means the gear meets a piston that has barely started moving, which invites premature engagement and stripped racks. Each tooth removed typically costs roughly 10 to 15 FPS, because the spring is compressed over a shorter stroke, so builders compensate with a stronger spring and then re-check angle of engagement, since piston rest position has changed. Treat two or three teeth as the practical limit for most builds: beyond that, feeding timing and tappet travel suffer, and gains shrink.

Sorbothane, neoprene, or felt: which cylinder-head pad material holds up best in an AEG?

Sorbothane holds up best, because it absorbs the piston-head impact and returns close to its original thickness, while cheaper pad materials slowly compress and stay compressed. Neoprene is a reasonable budget option but takes a permanent set over time, and plain felt compresses fastest of all, quietly changing piston rest position as it thins. Whatever material you fit, treat the pad as a wear item: at every gearbox service, measure or compare its thickness against a fresh pad, check for cracking, hardening, or a pad that has gone visibly thinner on one side, and replace it if it has. A worn pad silently undoes your angle-of-engagement correction.

What does correcting the angle of engagement with a sorbothane pad actually protect in an AEG?

It protects the piston's first pickup tooth and the sector gear. Angle of engagement describes where the sector gear first meets the piston rack. If contact lands on the tip of the first tooth instead of its face, that tooth takes the full load at an angle and eventually shears. A sorbothane or neoprene pad on the cylinder head both cushions impact noise and spaces the piston back slightly so the gear engages the rack squarely. Check engagement whenever you change the piston, cylinder head, or gears, since any of them shifts the contact point.

Why does a high-torque motor suit a heavy spring build better than a high-speed motor?

A high-torque motor suits it because the limiting factor in a heavy-spring build is starting torque, not peak revolutions. Stronger springs (M120 and above) load the motor hardest at the start of each cycle and during semi-automatic shots, where the motor starts from rest every time. A high-torque motor turns that load with less heat and less battery strain, while a high-speed motor geared for revolutions per minute will feel sluggish, run hot, and can overspin on lighter springs. Match motor character to spring weight first, then fine-tune rate of fire with gear ratio.

What is overspin, and why does it appear after fitting a faster motor or a stronger battery?

Overspin is the gearbox completing more than one shot's worth of rotation per trigger pull in semi-automatic, so the piston ends pre-cocked or the gun double-fires. It appears when the drivetrain gains momentum: a faster motor, a higher-voltage battery such as moving from 7.4V to 11.1V, or lighter gears all let the cycle coast past the point where the cut-off lever stops it. Fixes include a stronger spring, an active-braking electronic trigger unit, or a slower motor. Treat overspin early, because repeated double cycles wear the cut-off lever and tappet plate quickly.

How much velocity change should you expect from moving one spring grade, such as M110 to M120?

Expect roughly 25 to 35 FPS per spring grade on a typical AEG, measured with 0.20g BBs. An M110 spring is nominally rated around 110 metres per second (about 360 FPS) and an M120 around 120 metres per second (about 395 FPS), but the rating assumes a perfect air seal and a standard barrel. Real guns vary by 10 to 20 FPS either side depending on compression, barrel length, and hop-up setting. Always chronograph after a spring change and leave headroom under your field limit, because a gun that is legal on a cool morning can creep over on a warm afternoon.

What does a rotary hop-up unit hold better than a standard dial unit on an AEG?

A rotary hop-up unit holds its setting better under vibration. Instead of a small friction dial that recoil and handling gradually walk off position, the rotary design uses a stepped wheel with positive clicks and a wider adjustment range, so the chosen setting stays put across a game day. It also makes fine changes repeatable: you can count clicks back to a known setting for a given BB weight. The unit itself does not add range; pairing it with a quality bucking and nub is what converts stable adjustment into consistent backspin.

When are ball bearings a better choice than solid bushings on AEG gear axles, and when are they worse?

Ball bearings are better on low-to-moderate stress builds because they cut friction, which returns a slightly higher rate of fire and less motor heat for the same battery. They are worse on high-stress builds: a strong spring and heavy piston load the tiny bearing races at every cycle, and a failed bearing sheds debris straight into the gear mesh. Solid steel bushings tolerate shock loads indefinitely and are the default choice for M120-and-above or high-torque builds. A common compromise is bearings on the bevel and spur axles with a solid bushing under the sector gear, which takes the hardest hit.

What does shimming an AEG gearbox actually adjust, and what does bad shimming sound like?

Shimming adjusts the height and lateral play of each gear on its axle using thin metal washers, so the three gears mesh with minimal friction and no side-to-side slop. Good shimming starts from the bevel gear's mesh with the motor pinion and works upward. Bad shimming announces itself as a high-pitched whine, a grinding or rattling cycle, a hot motor, and a drained battery, because the drivetrain is fighting itself. If a freshly built gearbox sounds worse than stock, stop shooting and re-shim; running it in will not fix geometry that starts out wrong.

Related guides

Airsoft Upgrades & Parts FAQ — upgrade paths and part selection

Airsoft Troubleshooting FAQ — diagnosing cycling, feeding, and accuracy problems

Airsoft Product FAQ — general product questions: platforms, specs, and how airsoft guns work