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 Case Report #1: Only Whirring Noise on a Cyma AK-47 (CM028)

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PostSubject: Case Report #1: Only Whirring Noise on a Cyma AK-47 (CM028)   Thu Apr 24, 2008 5:19 am

This is only a suggestion, but I thought I could start a series that I would call "Case Report" where we could share airsoft problems and solutions with each other that we can learn from. Often, those problem are repetitive.

Case reports are common in the health care field and provide a great arena for discussion. The main difference between a Case Report and the general repair/upgrade talk is that the former is well structured and you as the author have completed the project from A to Z. The end of the study would contain your thoughts and conlusions and would open up a discussion for everyone else.

I *suggest* the following format:
A) History and Description of Problem: Contains background information how this problem occurred (only facts, no deductions)
B) Findings and Conclusions
C) Discussions
D) References: optional; sources you used to get to your results

So if anyone of you could share his or her experience "with pictures" about any of your projects, that would be awesome. I'm sure a lot of "newbies" could learn a lot from the experiences of their team mates.

Let me start with the first Case Report.

Case Report #1: Only Whirring Noise on a Cyma AK-47 (CM028)

A) History and Description of Problem

"Conflict" acquired his Cyma AK-47 in a used condition and encountered electrical problems after a while, ie. the gun would not shoot at all unless he was fumbling around with the battery. But several weeks ago, the gun would not shoot, although the motor did audibly whir. It was a high-pitch whir, and Conflict asked me to take a look at it because he suspected that the piston might have stripped. He even got a hold of a temporary and used piston.

B) Findings
I had to remove quite a few screws until I got to the gearbox, which BTW is a v3. Shockingly, nearly all the screws were in a loose condition already -- those holding the butt stock, the foregrip, the handgrip, and the selector switch! Only the screws that secured the gearbox were tight!

Upon opening the gearbox, the following situation presented to me:


You can see that the spur gear has been stripped of quite a few teeth! Some of the teeth that were scattered in the GB are circled in yellow. The brand of the gear is marked as "XYT."

I replaced the damaged spur gear with a new Element steel spur gear. Here's a comparison:





Shimming
As you know, shims come in 0.2, 0.3, and 0.5 mm thickness to fill the gap between gear and bushings as needed. But I discovered that the shims that were used here were all of the same thickness, namely 0.3 mm as verified with a caliper. It appeared to me that the spur gear was shimmed a little bit too tightly. The bushings looked like aftermarket bushings, but I cannot confirm that. I simply took out the shim under that gear and did not do any further shimming because of time constraints, but the gears were spinning freely with only little horizontal spiel.

Selector plate



When I pushed the gearbox out of the gun's housing, a piece of metal dropped of the gun, and it was appearant that the selector switch was already broken as visible in the above picture. Interestingly, this did not impair the gun's function to switch between semi-auto and full-auto!

Electrical
It turns out that the loose contact of the gun was caused by a worn out mini-Tamiya connector. Unfortunately, they are notoriously known for this after an extensive use. The best long-term solution is to add a high-quality connector such as a Deans connector (also called "T-connector"). This is planned to be done soon.

Spring
I chronoed the gun at an amazingly consistent 271 FPS w/ 0.2g BBs and hop up completely disengaged. This suggests an M100 spring that has already been used for a while and/or a minor air leak. An upgrade to an M110 spring resulted in an increase to 321-323 FPS. Since I *know* that this spring can shoot 350+ FPS, the lower FPS is now traced back to an insufficient airseal.

Barrel
This is were I started laughing: When I took out the barrel for inspection, I found a huge piece of paper (probably 1.5 inches long) stuck in the vicinity of the hop up. Obviously, someone (if not Conflict himself) tried to clean the barrel and lost the paper. There was no way to remove the paper simply with an unjamming rod -- the paper was *stuck*!

The barrel per se was not as dirty as I expected, so it was either well taken care of or the gun wasn't used as much. A few shots of silicone oil and several strips of cotton later, the barrel was ready to be re-installed.

One thing I corrected was the rotation of the barrel inside the hop up system: When I was looking through the barrel I could see that the nub inside the rubber sleeve was not properly aligned horizontally. But a slight twist on the barrel counter-clockwise resolved the issue.

C) Discussions
The gun is up and running again, and this is exactly what Conflict asked me to do. I already shared with him that I did not overhaul the complete gun -- eg. to do a nice shimming job, or to take apart the hop up system and soak up the hop up sleeve in soapy water, or even to polish the barrel and gearbox sheels -- because this gun was simply not worth my time. After all, this has been a free repair and upgrade and will be returned to the owner in a better condition than before.

In order to keep the screws tight, I applied some Gorilla Glue on the threads, which actually works very well. I admit I couldn't find my Locktite.

The main question for this case is why the spur gear got stripped of a huge section of its teeth. Ad hoc, I can think of four possible reasons:

1. Too powerful motor.
2. Too powerful spring.
3. Material weakness/fatigue.
4. Poor shimming.

I think we can dispel points 1 and 2. The ROF with this motor is "average" at best (no exact figures available at this point) and an M100 spring can't really be considered as "powerful" in our circles.

Material weakness or fatigue is certainly plausible, but how likely is it that such a large section would fail? Maybe the failure did start with one broken tooth and then extended to a larger area.

Poor shimming could be a major contributing factor because a tight gear does increase the stress of all components in the gearbox. It is interesting to note that exactly the failing gear was the tightest of all gears.

Final thoughts: When replacing even just one gear, some people recommend to replace the *complete* gear set with the *same* brand b/c supposedly, there are minor differences in cut, and well-designed gears are fine-tuned to work smoothly with each other. As you can see in the above pictures, the teeth of the Element gear are indeed shaped slightly differently than the XYT. I would not assume, though, that the manufacturer of this Chinese-made gear (Element) has put too much thoughts into fine-tuning his products Wink.

My impression about the Cyma AK-47 (CM028) was so-so, although it's certainly a nice beginner's weapon. The motor was an average performer even with 9.6V, the plastic felt cheap and creaked. Then again, I shouldn't expect any surprises either considering at what price this gun is being sold (for less than $100).

Addenum: After replacing the worn out mini-Tamiya connector with Deans, the gun became quite responsive and the ROF increased noticably! Therefore, my comment about the no-name motor does not apply anymore.

It is rumored on the net (notice the word "rumored"!) that any Cyma model below "030" should be avoided due to construction and quality problems. I cannot really confirm this rumor because when I look at this specific gun, it seems like it has gone through quite heavy use and abuse and maybe it was simply time for something to break. But that statement of course, is speculation and open for "discussion," especially since I neither can put an age on this gun nor can I confirm how many rounds it has shot.
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CONFLICT
Gunnery Sergeant
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Posts : 50
Join date : 2008-03-03
Age : 25
Location : Puyallup, WA

PostSubject: Re: Case Report #1: Only Whirring Noise on a Cyma AK-47 (CM028)   Fri Apr 25, 2008 4:41 am

Quote :
- because this gun was simply not worth my time.
Im turning 16 this upcoming wednesday so I will be able to get a job and finally start
putting money into my loadout.
This is the gun im planning on getting, hopefully it will be worth your time if it
decides to break!Laughing
http://www.airsoftgi.com/product_info.php?products_id=3025

I was also wondering if its possible to replace the mag-well in this gun with one of an M4
as brice did to his G36?

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PostSubject: Re: Case Report #1: Only Whirring Noise on a Cyma AK-47 (CM028)   Fri Apr 25, 2008 7:00 am

Holy cow! Do you know how huge that thing is? Check this out:

http://www.youtube.com/watch?v=aQX_l07St1Y
http://www.youtube.com/watch?v=-836BVYtN9k

It's only a few inches short than an M14! A few other things to ponder about:

- The receiver is plastic and may wear off rapidly if you don't take pre-cautions (although I don't know how long you plan on using it Wink);
- AFAIK, there is no magwell converter to accept M4/M16 mags; the gun-type is too "new" on the market;
- What will your gamestyle be? You seem very active and like to "hit and run." In that case, a smaller, more compact gun may be a better choice.

If you know you want it, then enjoy it! Just pray that until then someone comes out with a disassembly guide (and perhabs there's already one out) b/c otherwise, I have no clue how to help you (unless it comes with an instruction booklet) *LOL*.

Jawz

PS: Please let me know BEFORE you buy it! Let me check if you could get it cheaper...
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CONFLICT
Gunnery Sergeant
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Posts : 50
Join date : 2008-03-03
Age : 25
Location : Puyallup, WA

PostSubject: Re: Case Report #1: Only Whirring Noise on a Cyma AK-47 (CM028)   Sat Apr 26, 2008 12:35 am

Jawz wrote:
Holy cow! Do you know how huge that thing is?

Of coarse, thats one of the reasons why I like it.
It may cost me some mobility, but I think its out-of-the-box condition it
should give me my moneys worth.

whats your guys opinions?

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PostSubject: Re: Case Report #1: Only Whirring Noise on a Cyma AK-47 (CM028)   Sat Apr 26, 2008 1:23 am

its gun number 4 on my list of things to get. i currently just bought number 2. m14 Very Happy Very Happy
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