Help! Cracked G&G gearbox V2 with blowback

[Daddy Foote]

I need some advice on a G&G TGM Q5 AEG Top Tech Advanced (Blowback - TGP-PM5-SD5-BBB-NCM) gearbox V2 with blowback rear wired. About 5 years old. After using full auto for about 20 rounds, had a T break then when I went to start another game it stopped working. I could hear a faint click when the trigger was pulled.
I took it apart (first time fun) the rail on the piston was engaged on the final gear, but shouldn't be. Also noticed that where the blowback piston connects to the shell, the shell was cracked so I need a complete gearbox or shell /case. So a complete V2 gearbox or a complete V2 shell.

Seams to be a weak point where the blowback piston attached to the main piston tube.

The gears and the piston rail also look worn. So I could get new gears.

What gears would be best?

Does the rail come off the piston or would I need a new piston?

As I'm already going to either replace the gearbox shell and swap all the bits over or get a complete new gearbox, what other simple upgrades could I do at the same time?
It already has a high speed motor in it.

I was thinking of putting a tracer with chrono on it. Are there some that talk to a mosfet?

I've put it all back together and it works again. But I'm guessing I'll have to do something soon or it could lock up again or worse.

Any advice is welcome.

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[Adolf Hamster]

hmm, that's the pneumatic blowback system.

the tricky thing is if you replace the gearbox shell for a standard shell there won't be any accommodation for the blowback components, so the interface for your charging handle and mock bolt cover possibly won't fit.

i've worked on the m4 variant of this box and i'll be honest i'm not a fan of that system, the blowback is a horrendous source of air leaks giving inconsistent performance. if it weren't cracked i'd be suggesting to de-activate it, which is fortunately not too complex you just replace the stock cylinder for one without the vent hole at the top, it'll lose the blowback but gain shot-shot consistency (aka accuracy), although you might need to drop the spring as the existing one will be slightly stronger than normal to account for the air used for the blowback.

if you can find that same type of shell, then that might be the easiest option, do the above de-activation of the blowback and run a lighter spring (will lessen the load on the casing making it less likely to crack again) whilst still allowing you to have the blowback cylinder in-place so it can interface with the charging handle/bolt cover. otherwise you might have to leave the bolt cover out, which is fine as it's just cosmetic but it won't look nice having the hop unit always on display. there's also the issue that another box might not quite line up right in the reciever/be subtly different and cause issues.

in terms of the gearbox internals, i'm wondering if what's happened is the system has somehow stopped just before release, then the motor hasn't had the torque to get it moving again when it's trying to start under full load.

hard to tell from the description if the piston/gears need changing or not, some pics might be helpful?

in terms of mosfets then it's a case of what sort of functionality do you want? is it just saving the trigger contacts from burning out or do you want fancy shenanigans like precocking as well?

 

[Speedbird_666]

Oof. The classic G&G cracked BlowBack GB shell.

I think G&G's idea of mitigating cracked GB's these days is to put a metal spacer inside the receiver (sitting around the hop unit) to 'brace' the front of the shell against the front of the receiver itself. I don't know if that's an option on MP5s though.

Replacement shells are available here: https://www.fire-support.co.uk/product/gg-version-2-gearbox-case-blow-back-version

I would replace the shell, then get rid of the gun personally as it's a known design issue, and not a question of if, but when it will fail again. Of course you can disable the BlowBack and follow Herr Hamsters advice, along with aggressively 'radiusing' the gearbox's cylinder cutout, which will help to some degree.

If you choose to use a non-blowback GB shell, I would suggest sticking with G&G to maximise the chances of drop-in compatibility.

 

[Tactical Pith Helmet]

@Adolf Hamster,is it possible to blank off the vent hole on the replacement gearbox shell?  Could one for example thread the hole and fit and grind flat a grubscrew?  It would keep the shell for fit, but lose the blowback nonsense.  

 

[Adolf Hamster]

@Adolf Hamster,is it possible to blank off the vent hole on the replacement gearbox shell?  Could one for example thread the hole and fit and grind flat a grubscrew?  It would keep the shell for fit, but lose the blowback nonsense.  

You can cutoff the vent hole by replacing the cylinder for one without the matching hole, the hole in the gearbox shell would then be redundant. Iirc you have to remove the rubber gasket as well but the rest of the blowback components you fit normally.

It'd still be a weak point for a future crack though

 

[Daddy Foote]

So it turns out  I have a few options.

1. Try to find a drop in magazine that's complete. With or without a blowback. One problem with this is a complete G&G magazine is like rocking horse sh#t to get hold of. Or pay £150 plus postage. 2nd Problem is that although all V2 magazines are supposed to be created equal, they are not. Slight size different and shapes with modifications from different manufacturers.

2. Get a V2 magazine from another manufacturer and wangle it in with the help of a Dremel or whatever. One problem with this is as fast as I find one it's out of stock.

3. Buy a g&g shell for an M4 and swap over all the bits from my old gearbox. Get an un-ported cylinder and piston with metal teeth. Or reinforced one from another manufacturer.

4. Get a g&g mp5 gearbox shell with the blow back. Swap everything out of the old gearbox and get a new piston with steel teeth.

Wish me luck. Lol.

 

[Adolf Hamster]

personally, i'd be going with option 3. @Speedbird_666's link has shells in stock. ofc wether or not you want to fix up to use or fix up to sell is down to you.

cylinders and pistons can be found here, along with other parts: https://www.ak2m4.co.uk

the steel rack pistons you'll want to glue the rack in place before final assembly (they're removable in case folk want to do any tooth shaving shenanigans).

if you're replacing the cylinder (and de-activating the blowback), the fps might jump up as a result, so be mindful a spring change might also be on the cards.