TM M4 Mechbox Jamming

[Ranger]

So here's the story:

I purchased my Marui M4 from rangers approximately 2 years ago now. I ran it stock for approximately 2 - 3000 rounds. I then purchased and attempted to install:

Unfortunately I did not keep packaging, so I am not positive what spring it was I finally decided upon.

a more powerful spring - perhaps it was M-130...


an air seal nozzle - wider/round hole i believe


a metal piston head - does this make sense?



So after installing these parts and reassmbling the gun I test fired the gun. About 5 rounds would shoot on semi then it would jam. The piston would rotate off of its guide and jam leaving the spring compressed. So I would attempt to align the gears properly, even according to an image that I had at the time. Anything I did would lead to the piston jamming.

I have tried many different things... disassembled/assembled the gun again and the story always ends with the piston jamming. I was in seek of help on these forums when this was all happening... but after many failed attempts I put the mechbox away and had to deal with more important issues. Now I have the AEG out again and wish to get it working so I can sell it.

Any suggestions? I have tried all I can think of.



I appreciate any help :salute:

[ILLusion]

FIRST of all, get rid of that metal piston head!!

With a spring that powerful and an aluminum piston head on your stock gearbox, you WILL crack your mechbox.

As far as the piston slipping off the guides... that's very strange to hear, especially if it worked fine before. Do you notice any excessive wear on the guides of the piston or on the rails inside the mechbox?

[MadMax]

Wow that's a new one for me too. I can't imagine how your piston could be rotating around. Does the mechbox close tight?

If you've somehow jammed something between the mechbox halves (say a wire) it might make enough space to derail the piston. Perhaps we're misunderstanding your description of your problems?

[mcguyver]

I've seen it happen quite a bit actually. Your spring looks like it's bowed like a banana. I went through a few PDI springs like that. With reinforced mechboxes and quality pistons, it would actually shave down one side of the piston and leave residue all over the mechbox.

In a Marui box with a stock spring guide and piston, I'm not surpised at all that the piston jumps around and turns out of the guides. Take your spring and throw it in the bush. It's toast.

Go to a better piston and use a 110 to max 120 m/s spring.

[Dirty Deeds]

He ran the gun stock, then upgraded, notice the metal bushings?

With that set up, be happy the piston jammed, and you didn't blow the front of your mech box out of the barrel.

First off, did you shim the gears properly? The other guys are right, lose the metal piston head, and buy an after market piston. Make sure you have a 120 or less spring.

A good purchase would be a bearing spring guide, a 110 spring, aftermarket piston, and a systema silent piston head set. Your mech box will thank you in the end.

Oh and make sure you shim the gears properly ;-)

[Ranger]

Originally Posted by mcguyver

Your spring looks like it's bowed like a banana.

Oh, the spring is still in mint condition... or not bent at least, that's just the shadow or angle that's made it look bent.

Okay, thanks for the quick replies. I suppose I can try and sell off these upgrades and go with the recommended parts. Any suggestions or guides that are helpful in the allignment and shimming of the mexchbox?

[MadMax]

Are you sure that your piston is actually spinning around it's centre axis?

Try assembling your mechbox without the mainspring, spring guide, or cylinder.

Install the piston and head and the cylinder head. It shouldn't be possible to push the piston forward far enough for it to ride out of the tracks in the mechbox if the box halves are completely closed. You can pull the piston forward if the cylinder isn't there and get a better idea of how it's coming out of the rails. You can also twist the cylinder so the side window is aligned with the piston rail to see if it's turning around.

There's supposed to be about 2.5mm of rubber pad in the back of the cylinder head. If it's missing, you might have enough space to push the piston forward far enough to derail the piston from the track. Other than that, I can't imagine how you have enough space to clear the tracks unless you have a very nonstandard cylinder, piston, or piston head.

The piston rails or mechbox rails would have to be incredibly worn for them to not provide alignment. I'm pretty sure they would outlast a lot of sets of piston teeth and sector gears.

A slight bend in a spring is not a significant issue. It's pretty common for springs to be slightly bent as they're removed from a mechbox. I'm not sure I've ever seen a mechbox fail because the spring was mildly bent. They're not very stiff or strong as beam members go. The spring guide and inside of the piston are quite able to keep a spring compressing straight (except if your stock plastic one snaps).

[Ranger]

double post woops
- Steel Bushing

[Ranger]

Alright, thanks a lot. I will give that a try.

I'm not too sure if any of this effort is even worth while now that I think of it. I mean I could end up with a working gun yes, but the reason for fixing it is to sell it for school money. I haven't any transportation and there aren't any representatives to verify my age anywhere even slightly close. End Rant -

So just to verify... I am planning on ordering the following from WGC. I wouldn't want to get my current setup working just to have it wreck my mech box because of the aluminum piston head and too powerful spring and everything.



HurricanE Tune Up Kit for M4A1 ( M120s )

Package Includes:
- Nozzle
- Cylinder Head
- Cylinder
- Spring
- Bearing Spring Guide
- Bearing Piston Head
- Steel Bushing