ACM piston AC99163 installed in Version 2 AEG gearbox

High-performance airsoft piston AC99163 UK upgrade

ACM 14 Steel Tooth Piston

£8.95

Full steel rack of 14 teeth for positive gear engagement
Fits Version 2 gearboxes used in M4, M16 and most standard AEGs
Suits both high-speed and high-torque upgrade builds
Smooth, consistent cycling on semi and full auto
Reduces misfires and stripped teeth versus worn stock pistons
Beginner-friendly drop-in replacement for first internal upgrades
Affordable spare to keep in your tech box

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Description
Specification

Rebuild your gearbox with confidence

The ACM 14 Steel Tooth Piston (ref AC99163) is a dependable replacement for the Version 2 gearbox found in M4, M16 and the vast majority of standard AEGs. A full metal rack of 14 teeth bites firmly into the sector gear, giving positive engagement and smoother cycling than a tired, rounded stock piston.

Built for high-speed and high-torque builds

Whether you are chasing a snappier rate of fire or a heavy-spring DMR build, a steel tooth rack stands up to the loads that shred softer polymer pistons. It helps cut down on misfires, stripped teeth and premature wear, so your gearbox stays healthy shot after shot.

An ideal first internal upgrade

Cheap enough to keep as a spare, yet trusted by experienced techs, this piston is a perfect entry point for players learning to service their own gearbox. Reuse your existing piston head and O-ring if they are in good order, or refresh the air seal at the same time.

Fitting and support

Confirm your gearbox is Version 2 before installing, take care with the compressed main spring, and keep your shims consistent. Not comfortable opening the shell yourself? Our Kent airsoft repair workshop can fit and test it for you, and you will find springs, gears and more in our airsoft accessories range.

Product information reviewed by James Smith. Every gun is checked & serviced by our Rochester workshop before dispatch.

General
Brand	ACM
Other
Type	AEG piston
Manufacturer reference	AC99163
Gearbox compatibility	Version 2
Compatible platforms	M4, M16 and similar standard AEGs
Tooth count	14
Tooth material	Full steel rack
Firing modes supported	Semi-auto and full-auto
Recommended use	High-speed and high-torque builds
Installation	Drop-in replacement

Recommended upgrades & accessories

Frequently asked questions

Which gearboxes does this piston fit?

It is designed for Version 2 gearboxes, the type found in M4, M16 and most other standard AEG platforms. Always confirm your gearbox version before ordering.

Is a full steel rack better than a half-steel piston?

A full steel tooth rack gives maximum strength and gear engagement, making it well suited to high-speed or high-torque builds. Many players still pair a steel rack with a good spring guide and bearings for best results.

Will this piston work on semi and full auto?

Yes. It cycles reliably on both semi-auto and full-auto, improving trigger response over a worn stock piston.

Do I need to replace the piston head too?

Not necessarily. This is a piston body with a steel tooth rack. If your existing piston head and O-ring are in good condition you can reuse them, though a fresh head can improve air seal.

Can a beginner fit this themselves?

Yes, with basic care. Gearbox work does involve a compressed spring, so work slowly and keep parts organised. If you would rather not open the gearbox, our Kent workshop can fit it for you.

ACM 14 Steel Tooth Piston