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Old 10-04-2013, 09:47 AM
CJ1993 CJ1993 is offline
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Your input needed today, please

Hi again, guys! For those of you who've never ran across one if my threads, I DO tend to post rather lengthy threads. So, if reading is not your thing, feel free to skip around and glance at the pictures that should be imbedded in this post (hopefully). I am writing this up on my HTC One, so forgive me if I make any grammatical errors. It kinda sucks typing up a story on a phone.

Anyway, my main weapon is my G&G TR4-18 light (Guay's version of the H&K-416 CQB) and I've been using it heavily for...I'd say the past 3 months or so? the first major modification I made was to disassemble and remove the blowback feature and replace the cylinder with one that did not have the hole for the blowback. Had I done my research thoroughly, I would have discovered that this was a bad idea. My theory is that the G&G blowback equipped gearboxes are ONLY meant to be used with that particular cylinder. It seems like most people who try to swap that cylinder out with a standard V2 have the same problem that I ran into. The gearbox shell snaps or cracks right at the front of the cylinder. So that gearbox shell had to go.

Fortunately, I managed to get my hands on a G&G gearbox that I assume originally came from a GR series m4. Good news, because that's a direct drop-in replacement with no blowback designs to mess with. So I swapped most of the internals because I wanted to keep the ridiculous amount of quality inside of most TopTech AEGs. Especially the trigger system and wiring (gold plated trigger contacts? silver plated wires? yes, please!). I DID swap the gears for some SHS-ratio gears, and also replaced the piston head (matrix polycarbonate for noise reduction) and piston (echo1 reinforced lightweight SHS). I was in a hurry to put the gun back together and get out on the field, so I have to admit to rushing the reinstallation. Any airsoft vets reading this are probably shaking their heads in dissapointment...I learned my lesson, I swear!

Not surprisingly, I began to run into issues. My battery life dropped significantly, the battery would heat up quickly, and the motor would get hot enough to warm my hand through the pistol-grip. These are classic indications of a bad shim-job. Well, I'm fixing all that today!


what she looks like now
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Old 10-04-2013, 05:16 PM
2tall 2tall is offline
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It sounds like IMO that the gearbox is not shimmed correctly.
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Old 10-04-2013, 10:43 PM
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If you haven't already, please check out Evil Head's tech videos on the G&G M4 Blowback system.

The comments are also quite valuable.

http://www.youtube.com/watch?v=0aZPNKLZawA
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Old 10-05-2013, 01:21 PM
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Could be either bad shimming OR motor adjustment.
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Old 10-05-2013, 03:25 PM
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I'd have to agree with the shimming. I found that when my gears were over shimmed, it created a lot of heat from the motors, wires and the battery. It also slowed down my ROF. I would suggest removing shims from the motor, but not too many. The gears should have a little bit of movement when the gearbox is closed but not too much. If you shake the gearbox when just the gears are in the shell, you should be able to move the gears freely and they should rattle just a little. (Correct me if I'm wrong, but I've never had gear issues this way)
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Old 10-05-2013, 03:42 PM
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The Blowback system uses some of the airflow generated to power its blowback piston. Without the box bleeding some of it air, the airflow goes straight into the nozzle. You have increased stress on your gearbox without the "cushion" the blowback function provides.

You have a damaged front end which is a standard V2 gearbox problem. Currently this is addressed by adding sorbo padding to the piston head to cushion the impact during fire.

If you look how the nozzle assembly attaches tot he gearbox, if the piston head slams against this assembly the stresses are transferred through the two knobs on the inner gearbox to the outer shell where the metal is thin. Since the metal used in most gearbox shells is not a solid metal but composite "monkey metal," a stress fracture develops from the vibrations generated when the piston head hits the nozzle assembly.

Here are a couple of choices to address this problem:
  • Buy a CNC Aluminum Gearbox shell - $$$
    This is a stronger consistent material that can withstand the increased stress. However, any problems in the gear box would hit the weaker link - strip gears, tappet plate, piston etc. But this would also mean you are not as limited by internal parts choice - you could toss anything you want int he shell and not worry about the front end cracking.
  • Pad the piston head with sorbo - there are guides on doing this on youtube.
    This is simply adding a piece of padding to your piston head or the back on the nozzle assembly(have not tried this myself, but am familiar with what it entails). The idea being the padding absorbs more of the impact from the piston so that less vibrations are transferred to the outer gearbox shell.
  • short-stroking your piston so it does not fully compress your spring
    This would be adjusting the timing of your gears and piston so that the piston does not fully compress your spring - therefore reducing the amount of force the piston is able to generate and lessening the vibrations transferred to the outer shell. You could then compensate for less power by making modifications to your hop and outer barrel.
  • use a less power spring and adjust for your lower power via modifications to your hop and inner barrel.
    This would be tossing a different spring into your gun. Less tension would be placed on the piston and no modifications would really be needed to gear or piston. Less power generated, less force transferred... blah blah blah...
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Old 10-06-2013, 05:02 PM
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The gearbox cracking issue is not related to removing the blowback system. Its simply because their is less material on the top front corner of the gearbox itself. I've seen cracked G&G blowback boxes that still had a fully functional blowback cylinder installed. Its a poor design and one of my critiques to G&G, but hell if that goes very far. Everything else in the box is usually fairly decent for any AEG on the market sans the spring-guide.

FYI, if installing a sorbothane pad, ONLY install it on the back of the cylinder head. Installing one on the piston head can potentially cause it to get snagged and jam. It will also increase the weight of the piston which only serves to exacerbate the original problem further. Properly rounded corners and a 1/4" 70 duro pad with protective covering installed on the cylinder head has been pretty effective in my experience at preventing blowback gearbox failures. You can use a dremel and small cutting wheel to remove a notch in a new cylinder so that it fits in the blowback gearbox if you wish to disable the blowback function. Just remember to de-burr the parts you cut so you don't destroy your piston o-ring.

After clamping down a gearbox in 4 points around the gears you are testing for shimming, a little bit of wiggle room on each gear is acceptable. The bevel in particular, since its easy to overshim it, in which doing so causes serious friction issues.
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Last edited by Spectre; 10-06-2013 at 05:04 PM.
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