What does that clicking sound mean? Spotting a stripped piston

What does that clicking sound mean? Spotting a stripped piston

your AEG was running fine and suddenly you hear a click when you shoot, or a rattling sound, and no BB comes out anymore. when your airsoft gearbox clicks there's usually just one thing going on, and that's a stripped piston. but not always.

this piece is symptom-first: what you hear and feel, and what it means. so you know what you're ordering before you open the gearbox.

first: safety and don't keep firing

if your AEG clicks, stop shooting. every extra trigger pull grinds away more material and can make the damage worse. battery out, magazine out, and only then keep looking.

the three main suspects

there are three causes that all give a click or rattle, and you can tell them apart by sound and behaviour.

1. stripped piston (by far the most common)

what happens. the piston has teeth on the underside that get pulled back by the sector gear. the last teeth, which load the spring the hardest, are polycarbonate on a stock piston. those wear or break off. then the sector gear grabs nothing and spins freely.

what you hear. a fast click-click-click or rattle, and the motor clearly spins faster than normal, because it has no load anymore. the sound is light, plasticky.

what you feel. the trigger feels easier than normal. no "push" from the spring.

extra clue. often it still fires the odd shot, when the gear catches a remaining tooth. irregular, then clicking again.

causes:

• a spring too strong for a stock piston
• 11.1 V on a stock gearbox (this is a classic, see my separate piece on that)
• wrong AOE (angle of engagement), so the tooth is loaded at the wrong angle
• just age and a lot of shots

fix. new piston. pick one with steel teeth on the last positions. and while you have it open: correct the AOE right away, otherwise you'll strip the new one too. see my workshop guide on shimming and AOE.

cost: a piston is 10 to 30 euro. the work is an hour once you've done it a few times.

2. stripped or broken gears

what happens. a tooth on the bevel, spur or sector gear has snapped off or worn round. usually the bevel gear, because it takes the hit from the motor.

what you hear. a heavier, more metallic sound. often one clack per cycle instead of a fast rattle. sometimes a maddening irregular sound.

what you feel. the trigger feels heavier or notchy. sometimes it locks up completely.

extra clue. with a broken gear you often feel a hard stop, whereas with a stripped piston everything runs too light.

causes:

• bad shimming (gears not running in line)
• wrong motor height (the pinion eats into the bevel gear, this is the most common cause)
• a metal piston head plus a strong spring, where the load is higher than the gears can take
• a broken piece of piston that got between the gears

fix. new gear set. and then fix the problem that caused it: re-shim and set the motor height. otherwise the new set is dead in three months too.

cost: a decent gear set is 25 to 60 euro. the work is more, because a full gear set means re-shimming.

important: if there are metal particles in the gearbox, everything has to come out and be cleaned fully. metal grit between your new gears is game over.

3. wrong motor height

what happens. the pinion on the motor and the bevel gear in the gearbox don't mesh at the right depth. too shallow and they skip, too deep and they grind and eat into each other.

what you hear. this is the subtlest of the three, because it's often not a real click.

• too shallow: a rattling, skipping sound that comes and goes, especially under load. sounds a bit like a stripped piston, but the sound clearly comes from the motor grip and not from the gearbox
• too deep: a high, shrill whine or grind. it still works, but it sounds tortured

what you feel. the motor grip gets warm. the sound changes when you adjust the motor height screw at the bottom of the grip.

the test that tells them apart. turn the motor height screw a small amount (a quarter turn) and try again. does the sound change? then it's the motor height. nothing changes about the clicking? then it's inside the gearbox.

fix. free. just a screwdriver. i wrote a separate piece on it, because this is the most skipped free upgrade and it prevents exactly problem 2 above.

others, less common

worn anti-reversal latch. the ARL is the little pawl that stops the gears turning back. if it's worn, it skips with a click and the spring can slam back.

loose gearbox in the shell. if the gearbox isn't tight in the receiver, the whole block can move and that gives a knock or tick. check the mounting screws.

a BB or broken part between the gears. often in combination with one of the above.

loose spring guide or broken spring. usually gives a rattling sound and a clearly weaker shot.

quick chart

the diagnosis without opening the gearbox

order:

1. battery and magazine out. always first
2. adjust the motor height screw, a quarter turn in, test, a quarter turn out, test. does the sound change? then you have your answer and it cost you nothing
3. pull the motor and look at the pinion. are the teeth rounded or broken off? are there metal particles on the magnet?
4. look through the motor hole at the bevel gear with a light. visibly damaged teeth?
5. turn the bevel gear with a long allen key. does it run smoothly? is something stuck?
6. shake the gearbox. do you hear anything loose?

if after these six steps you still don't know what it is, the gearbox has to come open.

prevention

most of these problems come from three things, and all of them are preventable:

• wrong motor height. free to fix, everyone skips it
• bad shimming and wrong AOE. costs you an afternoon and a set of shims
• 11.1 V on stock internals. don't do that without a MOSFET and a decent piston

an AEG that's shimmed well, with correct AOE and the right motor height, lasts years. one that isn't comes back to the bench every season.

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