So way back in the day when I bought my first airsoft AEG (A little less than a year ago now, since the law in Ireland has only recently changed), I bought a Cyma Mp5-J from actionhobbys, I think it cost me around €130 at the time, which is an average price for a clone. Service from actionhobbys was very good, I think I was one of their first customer’s from Ireland and they were very helpful in looking up shipping costs for me. Anyway, being my first AEG, I loved it, but even now that I’ve bought more airsoft guns and fondled plenty of other people’s, it is still a great gun for the price. It is well made, sturdy and I’ve yet to have any major problems with it. It fires at around 260fps with 0.2g bb’s, which is decent enough.
Since it didn’t cost too much, and I’m currently using a TM P90 as my main aeg, the mp5 has become my project gun. I think I must have stripped and rebuilt the gearbox at least 20-30 times now, and that’s no exaggeration. The first of my many upgrades on this gun was a Systema 6.04 tightbore barrel, one of the downsides of the Cyma is that the hopup unit is a little weird and non-standard, so a bit of dremel’ing was involved to make it fit, but this wasn’t too hard to do, taking about an hour or so to complete. All went well and I brought it to my next skirmish the following weekend at hrta, before the first games of the day, I was out in the firing range and had my first experience of “THAT horrible grinding noise”, yup I’d stripped a gear, I cursed the gun and ended up using a Galaxy mp5k for the day which was a rental gun, I also had my HFC m190 pistol at this stage, but not the P90.
Eager to get my mp5 back up and running, I ordered a Guarder Full Tune Kit from ehobbyasia. I had a few problems getting the FTK installed in the Cyma gearbox, it would often jam up after several shots or jam before even firing one shot. After several days of frustrated rebuilding, I eventually found the problem to be the Guarder upgraded piston, I swapped out the piston with the original Cyma one in the gearbox and had the mp5 up and running quite well for a while with the stock spring and piston and the rest of the gearbox upgraded. At this stage, I’d had my p90 a few weeks and the mp5 became a little less used, it was under-powered and nowhere near as fast as the p90.
About two weeks ago I decided it was time to give myself a reason to use the mp5 again. I went ahead and ordered a PDI 120 spring, which is good for 300-330fps and also a Systema Turbo motor. I’d read some information on the Systema Magnum and Turbo motors, they offer high speed and high torque in the one motor, ordinarily it’s a choice between one or the other. They are pricey motors at approx USD$80 without shipping, but I figured why not? While I was browsing, I decided I’d throw a 9.6v battery into the order for good luck, I was still using the 8.4v that came with it and needed a new battery anyway.
Last Friday I installed the new motor and spring with the rest of gearbox as it was, holy shit are those motors fast. With the stock motor and spring in my mp5, I was getting around 12/13 rounds per second, with the new motor and a 1 Joule spring it was pumping out an amazing 24 rounds per second, pretty damn fast I have to say.
After my initial excitement about the rate of fire, I started actually listening to the gearbox between shots, it didn’t sound good at all, I knew what the problem was at once, the stock piston just couldn’t handle the upgraded power and speed, I left the piston in and decided to go ahead and bring it to a skirmish at hrta the next day. After some initial showing off of the rate of fire (people have commented on the speed of my p90 at 17rps), I attempted to chrono the mp5, unfortunately the semi-auto has never really worked all that well since I got it and no longer works at all, so firing a single shot is a matter of being very quick on the trigger, it was impossible to get a proper reading. I also noticed after my showing off that it didn’t seem to be firing very well at all and the gearbox noise had gotten worse, I left it in my bag knowing I would have to rebuild it when I got home.
Got around to opening the gearbox tonight and it was not a pretty sight at all. Eleven (yes, that’s right 11) teeth are stripped off the piston, and the next one is not looking too good either.
The sector gear didn’t look too good either, a few of the teeth (around 11 I would guess 
) had the gaps in between them filled in with a collection of the plastic from the piston and the grease in the gearbox.
I spent a few mins cleaning the gunk out from between the gear’s teeth and grabbed the upgraded piston that came with the full tune kit, I had decided I would dremel the piston if necessary to fit it in, the problem appeared to be caused by the rails not sitting correctly in the gearbox, I had a look and it seemed there was a bit of plastic which had been badly formed on one side of the piston rails, although it was barely visible to the naked eye, I could feel it if I put the piston into the rails inside the gearbox. A few rubs with a sharp screwdriver and the extra bit of plastic was gone, the piston now slid freely inside the gearbox.
So fast forward a half hour and I had the gearbox rebuilt, re-greased, the PDI 120 spring installed and the gearbox mounted back into the lower receiver with the motor installed. Now where the hell did I leave that bloody battery… 
They say a picture paints a thousand words, so below is a video of it firing, at this stage I was just testing so just had the gearbox mounted in the lower receiver:
As you can see, it’s pretty fast, with the new piston in, it’s getting 20 rounds per second now, still not too bad at all, the reason it has slowed down is because now all the piston teeth are intact, it’s not releasing the piston early anymore, if I was to remove a tooth or two on the piston, the rate of fire would increase again, probably back up to around the rof it was with the original (damaged) piston in place.So there you have it,
Cyma mp5-j = Great clone/starter gun
Systema Turbo Motor = Bloody fast motor
Cyma mp5-j + Turbo motor = CQB Support Weapon 
Now I just have to find a box-mag for an mp5.
Oh and incase anyone asks, yes the original mag can handle the higher rof just fine, no jams or misfeeds with it yet. Below is a video of it firing at a target net.