MK CNC Build - first timer

Hi all.

Bought a Kublai M4A1 a little while back as my first blaster. Pulled it apart, as you do, to see how things work, put it back together successfully, then decided to build my own gearbox from scratch.

Thing is, I live on an island in the Torres Strait and shit takes time (read: f’n ages) to get here, not to mention I can’t go and chat with people in stores to get advice. If something breaks or doesn’t fit, it’s a good 4 weeks before I can replace it.

I bought the MK CNC box and am trying to figure out what quality parts to put in it. I figure I may as well spend some money if the box is able to take it (otherwise I’d just upgrade the nylon Kublai box). Without spending $200 on a set of gears or $220 on a motor, what are:

  • good strong gears to get (I’ve heard there’s some shit metal ones out there. I’m thinking BigRRR 16:1 or 13:1)
  • Quality piston/Cylinder/nozzle etc combinations?
  • Motor that matches 16:1 or 13:1 gears

The problem I’m having is that it just seems that you have all of this stuff to choose from, and from what I can tell - you just roll the dice and hopefully it’s not crap and fits?

Learning what I can from the forum and YouTube, so appreciate any help or advice or recommendations!

Maybe mod the other box first, what I mean is there’s a lot to modding a gearbox correctly.
Anyone can throw together a box, but a great build requires a bit of experience. That’s an expensive MK box and you really should mod something cheap and nasty to gain experience.
My 2 cents


Mate, fair call. I just bought the box because I’m out miles from the mainland and and figured I may as well go hard, given how hard it is to get things here.

Apart from shimming the gears correctly, what other things should I do to level up in my gearbox building skill-set? I can imagine it’s about how well things work together as a whole, and not just the sum of all of the parts.

Unfortunately there is a lot of trial and error involved… and the errors make the best learning experience.

When you are talking about 0.1mm making a difference between working and not working you’re totally right about things working as a whole.

This nozzle works perfectly with that cylinder head, if you use this particular cylinder, but if you use this other cylinder then you have to use this other nozzle but the same cylinder head as the first one.

and so on…

I’d say that’s half the fun, learning what works. But waiting for things to arrive only to find its the wrong part sucks big time.
That’s why I always have a few projects on the go. while you’re waiting for one part you can be making something else…

I know that doesn’t really help you though…

You’re right about a few projects on the go (that’s why I got the Kublai in the first place); I’ve got a ukulele, lino cutting and printing and a Fat Bike to pass the time (not to mention 2 kids). You can’t have car or motorbike hobbies up here, and if you did - everything would just rust.

I will take up the advice to upgrade the Kublai and slowly build the MK from what I learn. I bought a same-brand piston/cylinder combination to throw in to the nylon box and see what happens. It was fun pulling the gearbox apart, but I’ll admit… It took 5 times longer to put it back together.

you could also try just putting all the Kublai gearbox internals in the MK gearbox and hand crank it. That will give you a good idea of what works with what, and what needs to be different

Or, hopefully someone builds a boss MK box and does all the heavy lifting for me! Save me taking trips down to the island “post office” to see if anything has turned up.

Off topic: I stretched the charging handle spring when I took the gearbox apart, and I can’t seem to find where to get a new one. Anyone know?

This guy: can’t pull the receiver apart without stretching a spring and then wants to build an MK gearbox :rofl:

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Those springs are crap anyway. You just look at them and they stretch. The j9 usually comes with a spare because of that.
I ended up putting a spring I got from bunnings on mine. If you use the replacement spring that’s the same, then it will happen again. And again. And again…
So don’t beat yourself up over that. Technically it was f*ed up before you even touched it!


even when you are experienced you’re still going to have those…ahhh fk moment…i did not just…friggin muppet…

second: practice on something sacrificial if you plan to do crazy shit in terms of mods and performance.

pulling down the kubbie…not a bad idea but you NEED to pay attention to every little itty bitty detail

putting it back together…power nothing up until you are absolutely certain you’ve tripple checked absolutely everything. this will save you crying in your weetbix because you stripped something…and we’ve all done it…been toooooo keen…

beyond that, low guido and the airsoft guy have vids that i have found very insightful

and there is always this brains trust…some fairly competent campers here



I reckon I’ve watched most of Guido’s vids. They go for ages, but are worth it!