I put about 20 000 rounds through mine stock before opening it to upgrade the spring. Having everyone tell me the potentially problematic parts when upgrading the spring, I bought all the replacements to install with the new spring. I got a Systema bearing spring guide, Systema silent piston & cylinder head set, Systema tappet plate, Systema polycarb piston and Systema nozzle. Basically, every part of the compression system except the cylinder itself.

When I got it opened, the only stock part that showed any sign of (very minor) wear was the spot on the tappet plate where the sector gear nub connects with it. But I swapped the parts nonetheless. The new Systema polycarb piston lasted all of about 200 rounds before blowing up. I reinstalled the stock piston and it's been fine since (about 5000 rounds since). The Systema tappet plate lasted maybe 500 rounds before it broke, and I reinstalled the stock tappet plate which has been in for about 4500 rounds now and no issues. Seems like the stock CA parts are more durable than "top of the line" Systema after-market parts (and yes, I got real Systema, and not Area 1000 parts).

You need everything - the trigger assembly, selector plate, cutoff lever, wiring, etc.

So I'm looking at a $400 box?

Systema drop in... cheaper and a pretty damn good gearbox.

It's gonna be pretty expensive if you're looking at top-shelf aftermarket parts in a Pro Win box. I'm still of the opinion that you'd be best off with a Systema drop-in. It will be just as monstrous, perform just as well, and be as durable as the Pro Win. The only "down side" with the Systema is that you don't have the split design to make spring changes easier.

But hey, it's your gun and your money. If you don't mind spending the cash and feel comforable making any necessary mods to fit the Pro Win, then it would be a great learning experience.

I highly doubt a Systema mechbox would be a durable as a one piece(or technically two piece, or whatever) CNC'd Pro-Win mechbox. But I agree with everything else you said.

Great thread, fantastic info guys.

Well, at under 400 fps, I think a Systema shell wouldn't be likely to break and would probably last the for many years, possibly even the lifetime of the gun, just as the Pro Win would. I don't disagree that the Pro Win could take more a lot more sustained abuse with a stiffer spring than the Systema though. I'm talking more in terms of practicality with his targeted velocitiy. They'd effectively be as durable.

The Black pro-win set comes with a non ported cylinder, bearing piston head, Brass spring guide( not bearing), 2 springs, cylinder head, metal nozzle, 2x hop rubbers and a few others..

The main ones you will need to add yourself are.

Gears,
Shims
Piston
Anti reversal latch
Switch assembly
switch cut-off leaver that the gear uses to make the gun run in semi auto.
Use a new hop rubber - i liked the systema one i used.

I think these are the main internal gearbox parts you require separatly. Get the right Cylinder you need for your guns barrel too, ( i took out the one that was previously used so it was correct).

Remem,ber the systema drop is box is alot cheaper compaired to the Pro-win setup. Also theM120 setups that Systema do ready made will bring you in at around 390-410 region if i recall.

Right. I was just talking more of a hardcore spring, or extended use with a high to hardcore spring.

Any gearbox will be "ultimate" if the gearbox is worked or build by a good gunsmith.

I see some of you guys recommend Systema Drop in gearbox so much but dont get your hopes to high. I have fixed a few clients aegs with Systema gearbox and guess what its not the "best" or "perfect" you expect it to be. Some are poorly shimmed, not enough lube in the bearing bushings, piston head O-ring and the gearset and seen a few with broken trigger post.

I have Systema drop in gbox on my own and instead dropping it straight into my project M4 I replaced the whole internals and re-worked everything with zero tolerance and I can compare and prove my gearbox is one of the "best" or "perfect" gearbox out there. :)

Even though the Systema Complete Mechbox is "Drop In" I would also shimm it myself, as should you. Why would you rely on someone else doing the job and taking their word for it when you can ensure it and do it yourself. Don't forget to change it to dean connections, you will get a higher ROF.

I guess I just taked it for granted and assume everyone whose even working on internals would keep it in mind that even a prebuild needs to be checked. Its true though, sometimes they need some work before they are good to go.

I didn't mean to direct the question directly at you T7, just generally to everyone who assumes Systema "Drop Ins" Or any complete mech box are "perfect" out of the box. Its true though, if you want the job done right, do it yourself, if you can't do it, find someone who can.

JG Complete Mecha Boxes require mandatory maintenance/preventative maintenance I find, Ive seen two so far, both required shimming and wiring ain't that great.

You probably learned alot about mechboxs having a VFC HK, eh T_A_N_K? I heard they have some issues.

OK. I'm ordering a Systema drop-in.

Which model do I get for 1200 rpm @ 375 - 400 fps with .20's.

What motor would work the best?

It has a Prometheus M4A1 (363mm) 6.03 TightBore inner.

I was thinking of a standard box with standard motor running off a 9.6v battery.

I'd like to achieve those figures with an 8.4v battery, as I have to build a custom battery because I have a DD 9.0 front R.I.S. and I'm getting a Magpul UBR RS stock which has no room.

I plan on placing cells wherever I can, linking them, and just having the connector somewhere I can access. I'll just plug the gun in.

Thanks for your help. I'm gonna get this thing done.