Gearbox is the central mechanical component in automatic electric airsoft guns (AEGs), containing most of the essential parts for the firearm's operation.

Gearbox parts are:

1. Set of gears (piston, sector, and spur gears)
2. Gearbox bearings (ball or slide)
3. Anti-reversal latch
4. Tappet plate and its spring
5. Cylinder
6. Piston with teeth
7. Piston head
8. Cylinder head with nozzle nose
9. Nozzle
10. Spring guide
11. Spring
12. Trigger and trigger spring
13. Mechanical trigger contacts or processor unit
14. Selector switch assembly
15. Mechanical trigger block and its spring (not necessary with some processor units)
16. Selector switch assembly spring (not necessary with some processor units)
17. Mechanical cut-off lever (not necessary with some processor units)
18. Wiring with battery connection and motor connection

The functioning principle of the gearbox lies in the fact that an electric motor drives the gears, which then move the piston back and forth. This motion compresses a spring located in the cylinder. Once the piston is in the rearmost position, it is released, and the spring pushes the piston forward, creating the necessary air pressure to propel the BB pellet. The trigger mechanism or processor unit controls the release of the piston.

Gearbox is a crucial part of AEGs and is often customized and upgraded by players to enhance the firearm's performance, reliability, and other characteristics.

Tokyo Marui sets several standards for gearboxes, including the following models:

V1 (first type in TM FAMAS F1 in 1992)
V2 (M4/16)
V2.5 (SR-25 and some HK417, extended cylinder and 19-tooth piston)
V2 ICS (split, slightly atypical)
V2 EFCS ARES/AMOEBA (for EFCS units from ARES and AMOEBA)
V3 (also includes cage motor and short motor for AK guns)
V4 (TM PSG)
V5 (TM UZI)
V6 (P90 and Thompson)
V7 (M14 and Sten AEG)
V8 (Type 89)

Followed by some atypical gearboxes:
TM NGRS EBB
M249/PKM
M60
AEP
ECSMG (AEP SMG)
T97
ARES LMG
L85
BAR1918
SVD
Real Sword SVD
PPSH
M3A1
MP40
and others

You can find images of all types of gearboxes here (link to a foreign website).

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