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KM4 RIS Gearbox (tight) fit into Lower Receiver


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#1 FlyingPenguin

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Posted 22 March 2014 - 03:42 PM

So I decided to tear down my KM4RIS today and do maintenance and such as it gets warmer. I had no problem with the disassembly, and had no problem with reassembly of gearbox and externals... but I can't for the life of me fit the gearbox back into the lower.

I try inserting the box at an forward angle, which lets me feed the trigger through the slot and work the box down... then it gets stuck about selector-plate deep. The bottom of the box won't feed through the pistol grip slot hardly at all, and the angle means that I can't get the rear of the box in the little set-back part near where the buffer tube goes. I've been trying for at least the last hour, with nothing to show but a few scuffs on the gearbox.

How do you guys do it? It seems incredibly tight, and even when I put pretty much all my weight in pressure on the top, it won't budge. It feels like there's no way it'd fit, though I took the box out of that exact spot mere hours ago. I'm getting so frustrated i'm tempted to file down some of the inner lips that stick out, making my life harder.. though I haven't yet and don't want to if I can help it.

What tricks do you have? Adjustments to make it easier? Am I just crazy?

#2 BLATEY

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Posted 22 March 2014 - 04:06 PM

Did you take the pistol grip off before re-installing? Pics too please, if you can.

#3 FlyingPenguin

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Posted 22 March 2014 - 04:12 PM

Yes, pistol grip was completely removed. It's quite literally only the lower, no anything else. I'll try and get some pictures.

#4 tales

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Posted 22 March 2014 - 07:26 PM

I reinstall the BB ramp on the lower receiver after i remove the gearbox, so i don't forget to install it when I'm reassembling the gun. Is your ramp on the receiver?

Pics will help!

Edited by tales, 22 March 2014 - 07:27 PM.


#5 gcw360

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Posted 22 March 2014 - 10:01 PM

Here are the steps...
  • Set the fire selector to semi
  • Angle the gearbox so the back side is slightly down
  • Insert the gearbox into lower so that the trigger comes through the lower receiver and set the front down so that the gearbox is completely inserted
  • Next, lift the front slightly to allow for insertion of the BB ramp
  • Complete assemble: note: put the body pin in first and work from there
You should not need to force anything.

Hope this helps.

#6 FlyingPenguin

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Posted 23 March 2014 - 08:41 AM

View Postgcw360, on 22 March 2014 - 10:01 PM, said:

Here are the steps...
  • Set the fire selector to semi

  • Angle the gearbox so the back side is slightly down

  • Insert the gearbox into lower so that the trigger comes through the lower receiver and set the front down so that the gearbox is completely inserted

  • Next, lift the front slightly to allow for insertion of the BB ramp

  • Complete assemble: note: put the body pin in first and work from there
You should not need to force anything.

Hope this helps.

So I tried this, and I pretty much get stuck with the trigger about a half inch through its hole. I feel like if i'm to push it any further, i'd need to use a lot of force. It just feels completely stuck. I've attached images of the box, lower, and about where I get stuck. It baffles me, frankly- it came out with no problem.

Here's pictures I took, let me know if you need more:
http://imgur.com/a/8LonI

#7 tales

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Posted 23 March 2014 - 10:53 AM

Try setting the selector switch from 12:00 (semi) to about 10:00 -11:00 move the gearbox gingerly forward and back.

Good Luck!

#8 FlyingPenguin

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Posted 23 March 2014 - 11:49 AM

Got it! Turns out the anti reversal latch wasn't 100% properly seated, so there was around a mm of extra space at the bottom of the gearbox, causing the tight fit. Retightened the bottom of the gearbox and it slid right in. Thanks for the help everyone!

#9 vanevery

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Posted 23 March 2014 - 11:53 AM

Great man. I was just looking at your pictures and you can visually see the gearbox shells not closed together completely in the picture looking up through the bottom of the lower receiver(4th picture down). Glad this is solved and you can move forward with a finished gun.

Edited by vanevery, 23 March 2014 - 11:54 AM.


#10 tales

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Posted 23 March 2014 - 01:47 PM

Glad you found the problem. You can see it in the 1st pic also. When i click on the pic if fills my 32" screen, great image site.
Good Catch!!




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