Upgrading Cyma M4 - Motor/gearbox sound issues

[Ben Savage]

Hi all, I'm having some issues upgrading a Cyma M4, with the sound coming from the gearbox being quite poor. I'll give quite an in depth description of how the issue started, as it wasn't doing this from the start.

 

I bought a Cyma CM603 for a low price in December with the intention to learn to upgrade and tinker with AEGs. It arrived firing 450fps on a 0.2g bb, so I needed to make some changes to it before I would be able to take it to a game.

 

I initially bought the following parts:

• SHS High Torque Motor
• SHS 13:1 Gears
• ZCI 14 Tooth Piston
• XT 6 Vent POM Piston Head
• XT Steel Spring Guide
• XT M100 Spring

 

I had to grind away part of the gearbox shell to fit the gears, as the Cyma gearbox is reinforced which prevented the spur gear from fitting properly. After doing this, I did my best attempt to shim the new gears. I attached the piston head to the piston, but forgot to use any thread lock when doing this. 

 

I then didn't have any time to continue for a couple of months, so I resumed the build this week. I found that everything looked good and ready to assemble, so I put everything back together, and it sounded like it was working great. I was achieving 20rps on a 9.6v NiMH, and it sounded quite good. I then was doing some more test firing, and suddenly a horrendous sound started happening so I stopped firing and disassembled the gun to find the screw holding the piston head had come out (Presumably due to forgetting to use any thread lock), causing a lot of damage. The piston, piston head, cylinder and spring were all damaged, so I purchased replacements.

 

I was unable to get the same piston which I had previously used, so I bought the following replacement parts:

• XT 14 Tooth Piston Swiss Cheese
• XT 6 Vent POM Piston Head
• XT Metal Cylinder Head (Original plastic one wasn't damaged, but gave a poor air seal)
• XT M100 Spring
• ZCI V2 Gearbox Screws (I had a screw head snap off the original ones during assembly, so I assumed they were all low quality so replaced them all)

 

After these parts arrived, I then reassembled the RIF and was hoping to have the same success as I had previously, but it sounded horrendous. I tried adjusting the motor height and it only sounded marginally better, so I assumed it would be the new gearbox screws making the gearbox close tighter, meaning I had to reshim. I then disassembled it and closed the gearbox back up with just the gears in, tightened the screws as they were before and tried to spin them, and they were quite tight with the new screws being used, so I decided to start reshimming (It is worth mentioning that the gears, motor pinion and piston teeth all had no signs of damage/wear). I then tried to shim it again from scratch, and after I was happy with it, I reassembled everything and test fired the gun. 

 

After reassembling it, I started test firing it and it sounded better than it did before, but it still sounded quite poor and very far away from how it did sound. I tried adjusting the motor height in both directions and I didn't really get any improvements, so I'm wondering what people think on here. I think maybe my shimming could have been done poorly this time, but before completely starting again I thought I would ask for opinions.

 

I've attached a couple of videos of how it sounded before the piston head came off, and after my repairs. Please let me know any thoughts on what the issue could be.

 

Any help would be appreciated, thanks.

 

Before breaking

After Breaking 

 

Edited March 23 by Ben Savage
Embedded videos

[Pseudotectonic]

SHS high torque is not high torque because it is just 16 TPA, I suggest getting 19+ TPA for 13:1 with m100

 

I would also inspect the gearbox for damage from the previous piston explosion

 

Could be shimming

 

Could be motor height

 

Could even be battery

 

Could even be new piston not sliding smoothly because it is aftermarket so the dimensions is a bit off

 

Could even be the grease or how much

 

Really need an inspection

[Ben Savage]

Thanks for the reply, with regards to the motor, I was considering short-stroking the sector gear by a few teeth, especially as I have bought a ported cylinder, so the last few teeth of the sector gear are not causing any extra volume in the cylinder. Would short-stroking reduce the need to go for a higher TPA motor as the spring won't be being compressed as much?

 

Also, I gave everything an inspection after the piston head detached, and anything which had even a slight bit of damage was replaced, so it doesn't seem to be any remaining damage that is causing an issue.

 

With the piston potentially being the issue, I am thinking of trying to swap the rack on the new piston with the old one, as the previous rack didn't get damaged but I am unsure about if it is a bad idea to swap the racks on pistons by different manufacturers. Although when turning the gears manually with the piston in place it doesn't seem to be catching on any teeth so I don't know if that means it would be unlikely to be the problem.

[Pseudotectonic]

Shortstroking does not reduce torque requirement much at all, yes it affects long term power consumption because it reduces torque requirement at the end of the cycle, but the peak power usage is at the beginning, so the peak power requirement (surge current) is unchanged (which is why you need higher TPA motor)

 

About the piston I was more worried the guiding rails on the sides, rather than the teeth

 

Also there is the question of AOE

[Colin Allen]

Assemble the gearbox without the spring, spring guide and sector gear. Once it is tightened up, hold the gearbox vertically to see if the piston slides down easily.

If it doesn't you can try polishing the piston guide rails, although you may need a piston with wider slots for the guide rails.

There is no point changing the rack.

It does sound like something is a little tight somewhere.

[Ben Savage]

44 minutes ago, Colin Allen said:

Assemble the gearbox without the spring, spring guide and sector gear. Once it is tightened up, hold the gearbox vertically to see if the piston slides down easily.

If it doesn't you can try polishing the piston guide rails, although you may need a piston with wider slots for the guide rails.

Ah thank you, that seems to be the issue - I did as you said and it required quite a lot of force to move the piston within the gearbox, I then tried the stock piston which moved very easily, and then the remains of the previous piston could move but not quite as easily as the stock piston.

 

I hadn't realised that the width of the piston itself could be an issue. I will see if I can do anything to make it move easily, otherwise I will buy a different piston and hope that it fits.

[ak2m4]

CYMA make pretty decent pistons, did the stock one have a full metal rack?  

[Sewdhull]

If you were to short stroke the thing you would have less energy to transmit to the bb since the spring will have less energy. If you kept the same spring then the motor would have less work to do and would cycle faster but the BB energy would drop, but perhaps not by much if you have plenty of volume in the cylinder compared to the  barrel.

 

Low TPA motors tend to draw a lot of current if they cant spin up close to their off load speed so starting from no revs draws lots of juice. You'll still get plenty of torque but at the expense of high current draw and a warm or hot motor.

 

I've not seen a tight piston, side to side, only one top to bottom with the sector gear installed, which is a rack height, gear size issue. I'd be checking the gearbox                                                                    piston rails are undamaged, or true.

I'd also measure the pistons to see what the difference is.

[Ben Savage]

4 hours ago, ak2m4 said:

CYMA make pretty decent pistons, did the stock one have a full metal rack?  

The stock piston had one metal tooth, and the rest were plastic so I thought it would be best to go for a full metal rack piston.

[ak2m4]

2 hours ago, Ben Savage said:

The stock piston had one metal tooth, and the rest were plastic so I thought it would be best to go for a full metal rack piston.

 

Ahhh right must be their super budget range, CYMA really do squeeze out every cent.  How much did you pay?  Saw these for under £90 a while back.

If it is the piston, might be worth picking up a CYMA piston, they're only around £8 I think.  Only downside is they are 15 tooth as standard so you'll need to shave down if you want to do any AOE correction.  

[Ben Savage]

2 hours ago, ak2m4 said:

CYMA really do squeeze out every cent.  How much did you pay?

I paid around £100 in December, with the intention to gut it and get a bit more practice at working on AEGs, some parts are shockingly poor, such as the gearbox screws and the body pin which holds the gearbox in which were both seemingly made of cheese from how easily the body pin deformed and how one of the screw heads snapped off before the screw was tight. It was a bit alarming that it arrived firing 450fps (especially from a UK shop) and I think that some of the cheap parts used were probably not suitable for such a strong spring and would not have lasted very long.

 

I have a couple of Cyma 002 M4 CQBRs which have the metal receivers, and I expected the cylinders/inner barrels to be the same, but the cheaper CM603 has what appears to be a red anodised cylinder and inner barrel, whereas the more expensive CM002 has what appears to be brass cylinders and outer barrels. I found the finish on the cheap inner barrel and cylinder to be better than I had expected, but I still plan on replacing the barrel with either an XT (If 380mm is available) or ZCI (although I've seen their quality is declining), and the cylinder I had used originally until the piston head screw scratched the inside after coming free.

 

2 hours ago, ak2m4 said:

might be worth picking up a CYMA piston

I'm thinking of ordering some ZCI 14 teeth pistons for the current build (as the previous one worked quite well before my mistake) and having a few ready for when I decide to work on the CM002s which I plan on doing the same work to, as they already have a tooth removed which will probably be better than me attempting to remove a tooth from a Cyma piston as I don't think I would be able to do a neat enough job to be happy with.

[Colin Allen]

12 hours ago, ak2m4 said:

CYMA make pretty decent pistons, did the stock one have a full metal rack?  

I rather like the CYMA pistons; they just do the job.

[ak2m4]

12 hours ago, Colin Allen said:

I rather like the CYMA pistons; they just do the job.

they're pretty decent for the price.  Wish I could get hold of them easily though.  CYMA are just super difficult to deal with for various reasons.

 

@Ben Savage ZCI is decent, give another one a go and let us know how you get on. 

 

I'm started to list the widths (guide rail to guide rail) of all pistons now in the descriptions.  Hopefully it helps a few people out. 

[Ben Savage]

This morning I had a delivery of the parts which I thought would get everything fixed, and thankfully the piston now fits much better. I also had a look at the new piston width info in the descriptions, and I was unlucky enough to have bought the widest piston that there was, so I likely could have tried a different XT piston.

 

The selector plate on the other hand doesn't seem to be a straight swap - I have lost semi auto, it seems like the dimensions are very slightly different between the rocket selector plate and the original, so I am wondering if I should try to remove material to make the circled parts (image below) the same size as I am guessing that is the main issue.

Spoiler

After reassembling, it still didn't sound as good as I had been hoping - and I realised that it will now be the shimming. The issue I have been having with the shimming, is since getting the new screws which are holding the gearbox shut tighter, the spur and sector gears are too tight from a single 0.1mm shim, so I decided to not use any shims for these gears. I'm now noticing that the spur gear is clipping the bushing for the sector gear, and possibly even the bevel gear as well. This is an issue as if I put 1 shim below the spur gear to lift it above the bushing, it then becomes difficult to spin. Do I need to buy low profile bushings to solve the bushing issue? Or is there an alternative solution which would be better (maybe tapering the edges of the gears)?

 

And the most recent thing to have gone wrong for this build, is the positive wire on the battery side of the trigger contacts, is hanging on by a thread. It wasn't soldered very well from factory, so I'm now trying to decide whether to get the soldering iron out if I can find it, or go ahead and buy a Perun hybrid. I'm leaning towards soldering it back on, as I don't think I should try fitting an etu + mosfet until everything is working.

 

Thanks for all the help so far, hopefully I don't encounter any more issues after resolving the above.

[Colin Allen]

1 hour ago, Ben Savage said:

This morning I had a delivery of the parts which I thought would get everything fixed, and thankfully the piston now fits much better. I also had a look at the new piston width info in the descriptions, and I was unlucky enough to have bought the widest piston that there was, so I likely could have tried a different XT piston.

 

The selector plate on the other hand doesn't seem to be a straight swap - I have lost semi auto, it seems like the dimensions are very slightly different between the rocket selector plate and the original, so I am wondering if I should try to remove material to make the circled parts (image below) the same size as I am guessing that is the main issue.

  Hide contents

After reassembling, it still didn't sound as good as I had been hoping - and I realised that it will now be the shimming. The issue I have been having with the shimming, is since getting the new screws which are holding the gearbox shut tighter, the spur and sector gears are too tight from a single 0.1mm shim, so I decided to not use any shims for these gears. I'm now noticing that the spur gear is clipping the bushing for the sector gear, and possibly even the bevel gear as well. This is an issue as if I put 1 shim below the spur gear to lift it above the bushing, it then becomes difficult to spin. Do I need to buy low profile bushings to solve the bushing issue? Or is there an alternative solution which would be better (maybe tapering the edges of the gears)?

 

And the most recent thing to have gone wrong for this build, is the positive wire on the battery side of the trigger contacts, is hanging on by a thread. It wasn't soldered very well from factory, so I'm now trying to decide whether to get the soldering iron out if I can find it, or go ahead and buy a Perun hybrid. I'm leaning towards soldering it back on, as I don't think I should try fitting an etu + mosfet until everything is working.

 

Thanks for all the help so far, hopefully I don't encounter any more issues after resolving the above.

Low profile bushings may well be the best answer to the shimming issue; they are pretty simple to change.

[Sewdhull]

Are you sure your gearbox case isn't bent?

Are you pressing in the bushings?

I know gearbox casings vary but none of this sounds right.

I'd be measuring the gear widths and comparing then to the originals, same with the bushings.

You can take material off the bushing surface to thin it down but there should be enough recess to accept the bushing.

[SSPKali]

Make sure the tappet plate isn’t being jammed between the sector gear and gearbox casing, I have had this after perfectly shimming the gears and then wondering why it sounded tight.