AK 74U Dive in and Mods

JM J12 AK74U - Plenty of Pew for your Penny

More of an SMG than an AR.
Good weight, slim, balanced, metal parts and heavy duty nylon and quiet for 250+ fps OOTB.

Impressive build, easy to strip although 4 bolts to hold detachable front end is overkill.
Everything is easily removable including plate that houses the mag contacts.
Trigger feels good.
Wiring is minimal and tidy.
25cm 7.5 plastic barrel
T-piece is custom.
1.1 spring.
Solid gearbox shell.
Large capacity mag held on by a lip and not inserted, fits firmly with no play.
Non-descript motor is adequate but will not pull anything stronger.
At current prices, this is plenty of Bang for your Buck.

See through gaps in battery cover (use plastic sheet provided).
Flash hider goes CW onto a thread adapter that is CCW onto 14mm outer barrel. Tightening one loosens the other.
Mag springs both bent and not making contact.
Shot selector is flimsy, needs to be pulled away from shell to move and is held on by a tiny screw under a little cap that comes off easily.
Pistol grip is nicely textured but child-sized.
Open sights are tiny and close together like some pistols and too far forward to be practical.
No pre-charge.
No mounting rails athough removable side-mount adaptor is provided.

Seal check although stock fps suggests it’s good
Metal barrel 7.2 or 7.3, length to be decided
Spring tidy-up plus gen9 spring guide.
Spring upgrade depending on effect of tighter barrel. Closer to 300fps is adequate
UV LEDs, naturally. :sunglasses:
IR laser
Motor upgrade
Stabilise barrel in outer barrel
Hop up. Have to remove unthreaded section of outer barrel front end as it gets in the way of most threaded hop ups.
16:1 nylon
Something about that shot selector. :thinking:


320fps 1.18 spring, ported cylinder, stock internals with bearing spring guide (dont think it was a gen9 guide maybe gen8?)

Gen11 grip
Metal dust cover
Metal fold stock
Metal mag
Timber hand guard
7.5mm barrel with ltd mp5 hop up fits in metal muzzle.
And its not finished.


Gave me a stiffy.


Did you change the motor?
I might go with this.


I’m going to be watching this thread with a giant, raging,…

… baited breath :stuck_out_tongue_winking_eye:

@Slick dude, that thing is epic. Brilliant work bro…


Stock motor with a 11v turnigy.

I see. I have a few scrap ones so still deciding on a 460.

In terms of build quality, the MST is better and cheaper (more metal used where it matters). However, it’s been suggested that performance is still better on the J12.

I simply couldn’t figure out how the lower handguard is removed on mine - everything seemed to fight me. Wanted to fit the ribbed MST version instead (put wood on the MST). Ended up getting a second MST, so no longer planning to do this, but would love to know the trick.

My MST (external mods only):


Was this a straight up swap? I have some donor J11’s kicking around :slight_smile:

Does this also mean the J12 grip fits nicely on the J11?


I didn’t pay full price luckily.

It is up there but not easy to explain.
Barrel is short plastic 7.5, spring is 1.1 and piston when tested does not seal.

Lower hand guard removal - updated.

  • Remove front sight by pushing out 2 pins under it. There are grooves on outer barrel for them so alignment is no issue.
  • Remove front outer barrel clamp. On the JM there is a lever that needs to be turned fully forward. It is also aligned using a groove on the outer barrel.
    The lowerguard should come free.

Does the rail mount get in the way of the folding stock?

How does your model perform?

Did you make any internal mods?

Front end and gearbox.

Gear box easy to open.
Slide clip on top, 5 allen screws (2 short ones), cap behind trigger, shooting mode gear, motor mount.
Bodyshell cover catch.

Shell is stiff
Everything stayed in place including trigger assembly and anti-reverse.
No excess grease
Quality tappet return spring
Piston o-ring does not seal
Dried blue gel inside cylinder.
There is a spring inside bottom of motor mount.

Replaced o-rings in pistonhead and cylinder cap
7.3 aluminium barrel is 8.6 OD so it is sleeved with 10.0 OD stainless. This fits snugly inside outer barrel and tightly in t-piece
Spring guide
Doubled holes in pistonhead
Fitted 1:16 nylons
Soldered sockets to motor wires.


Got mine from here 10$ maybe can find it cheaper. Https://a.aliexpress.com/_dY1izHi

Fits fine needed a washer under the screw.


Yes :frowning:
I have an over/under folding stock, and this won’t work with the mount either…

Haven’t chrono’d - need to get off my arse and soak some gels

Untouched internally.

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I’ve done all the usual things to my J12;

  • Replaced Piston O-Ring, as has already been said, the original seal was… well, not.
  • SHS M85 Spring
  • O-ringed nozzle
  • 75% cylinder
  • ChiHai Blue Motor
  • Mag Prime - which is wired and mounted in the factory-designed provisions, which I was most excited about until upon testing realised the charging handle does not actually travel that far back… so I will readdress with a different switch type.

Shoots good, sounds good, but the magazine feed is shite and does not feed my milkies reliably. I have some more mags coming so will see if it’s an isolated issue but I will likely shim the feed channel in the mag anyway.

@Cruise Please tell me more about that barrel you’re using? Sounds like it’s what I need for an asoft outer barrel I have. PM if necessary.


It’s nothing special.
I had some 7.3 barrels that were 8.6 OD.
The t-piece was a typical 9.5 ID while the moulded outer barrel was 10.5mm ID.
Some sort of sleeving was needed to secure the barrel to the t-piece. I happened to have some 10mm stainless tubing with 0.5 walls.
I could sleeve the barrel to fit the t-piece and then tape the rest of the barrel to limit movement in the outer barrel.
I just decided to sleeve the lot from nozzle to muzzle. :sunglasses:

Could you use a switch at the start position of the charge handle? Say a microswitch, using the normally closed pins so the switch is open circuit until the charge handle is pulled.
I prefer to locate a button reachable by the trigger finger.

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Custom piston
The piston is custom with a tab sticking out the side to activate the blowback slider.
Have to decide if that’s important enough should I ever change to metal gears and want a metal ladder.
It also is set up to have the sector gear release just as the piston touches the end of the gearbox.

Logic tells me to play safe and lose a leading tooth on the sector gear. Decisions, decisions.

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Can get these metal ladders

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  • Removed leading tooth from sector gear. The piston almost bottoms out at full shot cycle and stock spring was sometimes jamming between piston and spring retainer.
  • Fitted 16:1 nylon gears
  • Added 'breathing holes to t-piece. ?
  • Cut top bar across the rear of the shell to enable access to spring without removing gearbox. Nylon shell is stiff and unaffected and there is another, lower crosspiece.
  • Cut small slot inside RHS so shot select pin doesn’t catch and require prying the shell away when removing gearbox.
  • Re-wired trigger to switch +ve line instead of -ve. Shot selector works the same.
    The trigger switch inside the gearbox switches the -ve line. Not convenient when a MOSFET requires the trigger to switch the +ve.
  • Added pre-charge button above trigger.-

    Just have to do some wiring. :roll_eyes:


Used mine native on a 3s and it ate the mags stupidly fast

A programmable burst selector chip would be handy.