Can a lower receiver be "too small?"

[Mr. G36!]

Hey, all. Quick question: Is it possible for the inside of an M4 lower receiver to be too tight to allow the gears in the mechbox to spin?

See, I replaced a Jing Gong M4 lower/upper receiver and pistol grip, and threw in a Systema mechbox. At first, the gears would just either squeal, sounding like the motor wasn't raised enough, but raising it eventually just locked everything, as it was too high. Now, I've checked shims and they're prefect - one thing I do know how to do. I tried different pistol grips to make sure the one I initially used wasn't just off on an angle or whatever, I checked the barrel and everything associated with it to ensure it was aligned properly and that it wasn't canting the mechbox slightly, and the battery is good.

Now I've built G36s from scratch and made works of art, but these V2 contraptions are still sorta new to me. I figured that I should ask before I start filing out the inside of the lower just in case that's just a really bad idea.

Thanks!

[R.I.T.Z]

the receiver shouldn't impact any of the internals..
this is strange.

[Spike]

Put the pistol grip and motor directly on the mechbox without the receiver and see if it works like that.

[L473ncy]

Maybe the lower is too small and making the gearbox shell bow in? Try spikes solution and if it doesn't make that sound then you can hypothesize that it has something to do with the lower and gearbox shell fitment. Either shave the outside of the shell or shave the inside of the lower. However that said, I've never heard of this problem specifically before.

I know that VFC shells don't fit into all guns (I actually needed to shave down a lot of the exterior of the VFC shell to fit it into a Madbull lower, namely that bolt release spring assy. thing and the small "post" that juts out of the shell).

[Styrak]

My suspicion: Both of the motor wires should run along the back of the grip. If you have them split, you're gonna have a bad time.

[Kos-Mos]

Originally Posted by Styrak

My suspicion: Both of the motor wires should run along the back of the grip. If you have them split, you're gonna have a bad time.

Untrue. It really depends in the make and model of the grip.

But it is not uncommon for the lower section of the receiver to be too low, causing the mechbox to tweak backwards and the pistol grip is installed.

Try the pistol grip on the mechbox without receiver first as someone said. It will sort out if the problem is from the receiver or not.

Then if it works perfectly outside, install the cross pin first, then the stock second and pistol grip last. Do not tighten it fully, and check if there is uneven spacing between the grip and the receiver. If so, your receiver is bad and you will have to shim and or file some of it.

[Mr. G36!]

Thanks for the suggestions. I had the gearbox mounted to the grip with motor to check my shims and everything moved nicely, but I didn't try actually running power to it so I'll do that next.

The reason I started suspecting the fitting of the lower just came to me today, when I tried firing in full-auto and then semi felt like the safe was set. Made me think that maybe the selector plate was being pinched somehow, or that possibly - though unlikely - the tightness of the lower might squeeze the bushings and gears just enough to put enough resistance on them to prevent them from turning properly, but the shafts/pins/whatever they're called would have to be really long to do that, and if the box goes in, it should have enough room.

I do remember working on an MP5 though that had a somewhat incompatible lower receiver, with notches and guides on the inside that stopped the selector plate from moving properly and filing those away allowed the gun to work again. I figured maybe something similar could happen here?

I'll recheck the box with the just the motor & grip and actually run power to it. Oh, and yeah, I've made sure that the motor wires aren't messing with the motor, so that's a box checked.

[ThunderCactus]

Stupid question, did you adjust the motor height?

And some grips do allow front/back wiring, but it's good practice and ALWAYS works to run both wires to the rear of the grip

Another issue that I've seen is shimming the bevel gear too far to one side. It would prevent you from getting proper engagement with the motor, it would sound like the motor is always too high in the grip. You want a max of .5mm on the small diameter end, and shim the rest from the large diameter end (the top)

[Eeyore]

Originally Posted by Mr. G36!

Hey, all. Quick question: Is it possible for the inside of an M4 lower receiver to be too tight to allow the gears in the mechbox to spin?

See, I replaced a Jing Gong M4 lower/upper receiver and pistol grip, and threw in a Systema mechbox. At first, the gears would just either squeal, sounding like the motor wasn't raised enough, but raising it eventually just locked everything, as it was too high. Now, I've checked shims and they're prefect - one thing I do know how to do. I tried different pistol grips to make sure the one I initially used wasn't just off on an angle or whatever, I checked the barrel and everything associated with it to ensure it was aligned properly and that it wasn't canting the mechbox slightly, and the battery is good.

Now I've built G36s from scratch and made works of art, but these V2 contraptions are still sorta new to me. I figured that I should ask before I start filing out the inside of the lower just in case that's just a really bad idea.

Thanks!

I find Systema gearboxes slightly larger than other brands. Mine barely fit into a KA receiver.

[Mr. G36!]

Well, I pulled off the Lower and attached the grip & motor directly to the bottom of the mechbox and and it cycles fine. Gears make a slightly louder sound than I'm used to, but I can't tell if it's because the motor height needed some slight tuning or if it's just the lack of a lower to soften the sound some.

In any case, it seems as if the issue is something to do with how the box and lower are mating. Now I looked in the lower but can't see anything obvious that would throw the box off, so I'm not sure what I might do, but it certainly seems as if it's behind the issue now.