Identifying the
gearbox
Identifying problems
■ AEG is cycling but air nozzle is not moving?
The tappet plate is probably broken.
■ Gears are moving, along with the tappet
plate and air nozzle, but no ‘pop’ from the
piston? The piston is probably broken.
■ Grinding sounds from the gearbox? Check
for incorrect motor height, poor shimming,
worn gears or possibly damaged bushings.
■ AEG still functioning but with a drastic drop
in velocity? The spring may have snapped (an
exceptionally rare problem).
■ AEG has been working fine but suddenly
stops and the motor will not cycle, and you
know that your battery is still charged? Then
the motor has probably burnt out.
■ Piston stopped moving or grouping and
range of BBs has become erratic? The O-ring
on the piston is probably broken.
■ Barrel jammed, and not due to a dirty
barrel or cheap BBs? The hop rubber may
have split, your hop setting may be too high,
or your hop unit has cracked (the latter is rare
and only found with plastic hop units).
Version 1: Tokyo Marui’s first gearbox is
found only in the Famas.
064
March 2012
There are several different variants
of gearbox, known as versions.
These range from V1 to V8, with
some being much less common than
others. Each version has parts that are
specific to it, and are not interchangeable
between versions. These include: spring
guides, tappet plates, air nozzles, trigger
mechanisms and motors.
The most common are V2 and V3
gearboxes. The easiest way to tell them apart
is by looking at the top of the gearbox; a V3
has a metal slide that holds both sides of
the gearbox together while a V2 relies on
screws to do the same job.
If you’re not sure which version you
have, compare the shape to those shown
on these pages.
Within the gearbox, and attached to
it, you will find the following parts: motor,
anti-reversal latch, bevel gear, spur gear,
sector gear, spring guide, spring, piston (with
attached head), cylinder (with attached head),
tappet plate and spring, air nozzle, trigger
mechanism, bushings (metal or plastic) and
shims.
Gearbox
maintenance essentials
Make sure you are wearing eye protection
before splitting the gearbox to avoid damage
caused by any small parts ejecting at
speed. It is also important to have all of the
necessary tools to hand.
When splitting the casing you need to
ensure that the spring does not eject from
the gearbox as it may snap the spring guide.
The risk of this happening in future can be
reduced by replacing the stock plastic spring
guide with a metal version.
It is important to keep the moving parts
well-greased but avoid over greasing, as
it leads to them moving slower,
jamming and draining battery power.
The gears should be correctly
shimmed as a lack of shims can
lead to gear and piston wear and
the tappet plate being impeded.
Use a grease of light viscosity (one
which appears clear or translucent
white). If using heavy based grease,
which is usually black or dark brown,
apply it sparingly. While you have
the gearbox apart you should check
for wear on gears and remove any
metal filings.
Version 2: One of the most common
gearbox types. Several companies have
redesigned and improved upon it, but
there are essentially three variants:
windowed, non-windowed and ICS.
Shown here is the windowed version (note
small ‘window’ to rear of gearbox) usually
found in Armalite rifles (which cannot
take a non-windowed type as it doesn’t
allow for the body tabs to pass over the
gearbox. The non-windowed type is found
in non-Armalites.
V2 used in: Armalites (CA, TM, G&G,
G&P, AK); MP5 (CA, TM, G&G, ICS);
G3 (CA, TM); M249 (G&P)
Do not force the parts if you encounter
resistance when putting the gearbox back
together. The parts may not be correctly
placed and continuing to force them could
cause unnecessary damage.
If you lack experience in opening
gearboxes always seek advice from your local
gun-tech, or check out some of the millions
of videos that deal with the subject on the
internet.
Version 3: Favoured for its reliability
and the abundance of aftermarket parts
available. Look for the retainer plate on the
top of the gearbox (instead of screws, as
used in V2). Also has a thicker shell.
Note: though SIG gearboxes can take V3
parts they are not normal V3 boxes – it
doesn’t have a motor cage, and features a
unique fire selector.
V3 used in: AK47 (GG, ICS, TM, CA);
AK74 (GG, ICS, TM, CA); UMP
(GG); Steyr Aug (TM, CA);
MP5k (TM); G36
(TM CA)