CYMA MP5 SD6 thoughts

[Sewdhull]

I made the mistake of looking at some of these falling down an internet rabbit hole and ended up hanging my nose over the gun in the title.

 

Of course I then bought one and following are my thoughts on the gun.

 

In case you don't know what it looks like... https://images.app.goo.gl/9mtVnfitABNswYUX6

 

For me it's a sexy gun, maybe the sexiest. Sexy and heavy. It weighs 3.6kg with mag, battery and sight and more than a little front heavy.

 

It's the Blue version, so inside it should have half decent bits and clearly the air seal is ok, firing at  365 fps on a .2g BB, through its little 220mm barrel. It's happy on a 3S lipo or a 3S life.

 

I swore to myself I would use it before taking it apart, but I didn't. It had a very clunky fire selector which bothered me greatly so I pulled off the sliding stock, which is very nice and sturdy, with a great extending action (only 1 of the 3 extended positions is useful to me, so I will mod those later)

Once the body pin was out, I slid out the gearbox out and pondered the fire selector.

 

I am used to a G36, they are V3 gearboxes and this is a V2. I don't recall working on a V2 before so of course I ploughed ahead, pulling the motor out and sliding the plastic bits off after taking the fire selector levers off ( grubscrew on the left, then pull the shaft and right selector lever through the gearbox.

There's a "plastic bit" that sits in the plastic lower, through which the selector shaft passes from the right and gets sandwiched by the right selector lever. 

 

Anyhoo, the gearbox has a quick change spring, a shiny ribbed cylinder and a plastic nozzle, sans o ring. The casting is textured for some reason, it's not radiused and not shimmed properly.

 

I noted that the plastic fire selector plate was binding leaving safe to semi on the contacts, which stuck up really high, so the copper part of the fire selector plate had a lot of work to do to get past them. I imagine it's meant to electrically disconnect the  trigger as well as mechanically lock the trigger on safe, but it doesn't do the electrical isolation. I will probably rewire this given the pointless contacts.

The fire selector plate and the aforementioned "plastic bit" ( a sort of cam) work together provide a crude detent, in addition to the detent in the left fire selector lever, when selecting safe, semi and auto. For safe the plastic bit has kind of beak that goes over centre on the bottom of the fire selector plate, on semi there's a nub on the plastic bit that nestles against a raised bit on the fire selector plate and on auto, the aforementioned beak engages the raised bit.

 

To smooth all this out required:

 

Pushing the contacts down ( they may have been incorrectly seated) so the fire selector plate would move more freely

Taper the beak of the plastic bit, so it would engage safe with less force and move from safe to semi with less force.

Reduce the size of the nub on the plastic bit to engage semi from safe more smoothly.

Taper the top side of the beak so it would more easily engage in the auto position.

 

There is still a detent effect putting the fire selector plate in the right place, which now works with the fire selector levers own detect, meaning it all clicks into place now. Much like my G36s.

 

Having, as previously mentioned, sworn to not take this thing apart I now have a list of things to do.

 

Spring change, maybe nozzle change.

Shimming and consequently adjusting motor height.

Filling in the 2 useless cut outs in the sliding stock so it slides to the right position.

Add a MOSFET ( nothing fancy, I have a load of 3034 chips to use) Some of the MOSFETs used in the posh ETUs are really not very good. Altho the Jefftron ones are very nice.

Rewire the thing in decent cable (the PVC stuff doesn't like the bends this gun seems to require and it uses  lot of it, going to the back of the gun from the gearbox then to the front to the battery)

Short out those weird contacts under the selector plate and get the copper off it.

 

That's probably enough for now, please feel free to tell me where I'm going wrong or what else I might do since I am already doing this ^^^

 

Maybe you've noticed there are not many pics, Some tech issues prevented them appearing here. I'll add them as soon as I can.

 

 

                              

 

 

[Sewdhull]

So I have filled in a pair of cut outs so the first position on extending the stock is a useful one.

 

 

I cleaned the slot, undercut it a tiny bit then used tape to hold in the epoxy on either side leaving the inner rail bit open and dribbled the expoxy in. I used JB weld. A little trimming with a knife took off any high points.

[Sewdhull]

Did the shimming and finished off the perun tasks today.

 

On this gearbox I removed the trigger screw protrusion with a drill bit instead of grinding stuff. The cutoff lever is removed with this perun hybrid.

All the scratches are from the manufacturer.

 

The perun sticker I applied after removing the copper and cleaning the plastic with isopropanol. 

The bearings are 3x8x3 and I had to widen the channel in the selector plate at the top in this pic to take some pressure off the plate so it moves without much effort. Even with 2.5mm ones in the plate rubbed

 

Bearings are from ak2m4, run nicely with thier 7 balls. Obviously better than 6...

 

When I'm finished opening the box I'll lube everything, seems I have a leaky nozzle, the blue, apparently 3d printed, one that came with gun has a texture on it, no o ring and a very strong tappet plate spring which I'll have to compare with some others before I'm happy.

 

 

 

[Colin Allen]

16 hours ago, Sewdhull said:

When I'm finished opening the box I'll lube everything, seems I have a leaky nozzle, the blue, apparently 3d printed, one that came with gun has a texture on it, no o ring and a very strong tappet plate spring which I'll have to compare with some others before I'm happy.

I recently picked up a new CYMA V3 gearbox for my MPiKM in place of the LCT one that was in it.  The blue nozzles are awful and the tappet plate springs do seem to be very strong.

[Sewdhull]

The spring in the cyma has .7mm wire, another from either the src or we has .6mm.

That's quite a significant difference and my mp5 doesn't have a rate of fire more than those other 2.

[Colin Allen]

4 hours ago, Sewdhull said:

The spring in the cyma has .7mm wire, another from either the src or we has .6mm.

That's quite a significant difference and my mp5 doesn't have a rate of fire more than those other 2.

That is a considerable difference; I didn't measure the wire thickness on mine, but 0.1mm larger seems about right and it was definitely much stronger.

[Sewdhull]

I bought a spring from ak2m4 and its .55mm, so way better in my view.

The .7mm has fewer coils too so it makes it even stiffer than just having fatter wire.

Tappet plates always look fragile, I guess it's the thin tail the fin thing hangs off.

[Sewdhull]

I have put the gearbox back together and test fired it and all seems to work nicely.

 

I have the ak2m4 motor 22tpa and 13:1 gears, fettled the box and set up the Perun hybrid. Very easy and intuitive once you understand the speed you need to flip auto and single fire back and forth.

 

I had no issues with the Perun, all worked as it should, I kept the manual safety so that the selector plate has some spring tension on it still ( the Perun removes the selector lever and therefore the plate spring )

 

I had previously fettled the selector plate etc to get smooth fire mode selection.

 

I have the std spring in at the moment, 11.1v lipo and the std setting for the Perun has braking on at 3 iirc, so I took it to zero, added 3 round burst on auto and played with the pre cocking.

 

As it turns out, with no braking and no pre cocking, 3 round burst pre cocks it nicely to the rear.

 

In single fire I needed pre cocking to 3 and braking off. pre cocking 4 gave me a 2 round burst, I'm trying not to use braking as I can feel the motor heating even with these settings and I am thinking a 7.4v lipo might be a better option as the rof sounds more than I'm after. I was thinking the motor might be slower than it is ( but it's not), so lower voltage seems like a good shout. Maybe I should have gone with the 28TPA... Hurry up with the brushless AK.

 

I have still to live fire the gun, check feeding and probably have to fiddle with the settings once there's a bit of back pressure and measure the fps. I intend to swap out the the spring with others that I have and see how they relate. Then it's barrel and hop time.

 

I have put the box together with a minimum of lube, the Perun uses optical sensors which I don't want greasy and it looks like a greasy sector gear would contaminate the sensors. I will strip it down after using it a while to see how the parts are working together. The pinion to bevel alignment looks good with grease on the right bits of the bevel. Many shims aren't flat when they are made (they re stamped and end up a bit curved) so all the shims I used i flattened on a block so they sit flat, I'll check those in due course.

 

The wiring goes to the rear now and I have a solid stock to replace the sliding one that came with the gun.

 

A few of the fettling mods,

 

Shimmed the motor tower to remove play into the gearbox. Brass tube

Shimmed the hop unit to remove play into the gearbox. Brass tube ( a ring really)

Reprofiled the selector plate and rounded the grub screw on the right side of the selector as it was rubbing on the handgrip.

Added xt30 connectors instead of the Deans.

Used a thinner wire tappet plate spring, for ak2m4, the original was verrry stiff.

 

The Perun wiring is 18AWG which I may well change if I feel the need. I have some 14,15 and 16 Teflon cable so i can choose. There are pads on the Perun to add cables for a tracer should I feel the need. I'd add a couple of LEDs to the hop unit if I did that. Already have some in the parts box.

I modded the feed tube to my g36 with an )O ring to stop the bbs dropping out, I may do this to the MP5, ill see what the timings are like but its not high on the list.

I've got some buckings to try and the silencer to fiddle with once I am firing bbs.

 

Edited January 31 by Sewdhull

[Sewdhull]

Gun running well, I need a spring. The following is with zero hop.

 

Barrel I believe is 229mm.

 

Unknown spring  325 FPS .2

Unknown 2 390

Begadi 110. 420

Xt 120  420

Xt 130 445

 

22rps 11.1v

 

I'll have put a connector on a 7.4v...

 

Or maybe short stroke it a bit...

 

Any thoughts on that?

Edited February 6 by Sewdhull

[EDcase]

Funny I've been eyeing up one of these even though I already have a CA MP5 and JG MP5K.

I was toying with the idea to get an SD to put a longer barrel in but duno if it will need one.

 

For your build the Unknown spring at 325 seems ideal.

Are you thinking of changing the hop rubber?

 

 

Edited February 7 by EDcase

[Sewdhull]

They are a very pretty gun the solid stock is my preference, the collapsing stock is nice for transport. Detach the silencer and it's an mp5k nearly. It's only one pin to remove the stock anyway.

 

Yes I have a rubber to add once I'm  ball park energy wise from ak2m4. And some others I haven't tested.

The unknown springs are from other guns.

I might lose some energy hopping .3 or .32 so I was hoping to be a lil closer to 350 and it's a short barrel so I should run a few of those through before I do anything drastic.

I'm likely to replace the 390 one in the gun it came from.

I've just taken the bearing off the spring guide to reduce the spring tension.

Also I'd really like to know the spring number should I need to replace it in the future.

 

I'll fiddle more tommorow 

 

I think the barrel is long enough but if you do make it longer your gonna need a weaker spring.

Also I'm not convinced the hop nozzle seal is right yet.

Edited February 6 by Sewdhull

[EDcase]

Will you use it mainly for woodland or CQB?

[Sewdhull]

Woodland, or at least open.

 

I've taken a spring bearing out and gone from 390 down to 372 to 375.

 

I'll order some springs tonight I think.

[Sewdhull]

 

312   xt95

324  xt100

356  Xt105

 

With the spring guide bearing removed.

 

[EDcase]

Personally I'd rather get a lower power spring or cut some coils rather than take out the bearing.

[Pseudotectonic]

Agree you need the bearing for consistency and reduce chance of the spring getting hiccups

 

For the 18 AWG Perun wires I actually appreciate they put this in, this is some very high wire count with silicone jacket so it's very flexible which is great for working with, and I've calculated the difference of ohm between 18 and 16 AWG for the lengths in airsoft is really negligible

 

Have you tried polishing the piston groove walls on the gearbox and where the tappet plate and selector plate slides on? I touched up mine with some 5000 or 7000 (forgot which one) papers and it does make things glide on the gearbox better (now I actually have a problem with an overpowered m90 spring shooing 355 FPS because it is too efficient but that is another topic)

[Sewdhull]

You only need one bearing. Piston end if you have a choice. It's the piston compressing the spring. 

[Pseudotectonic]

but when the spring compresses the whole length (every coil) rotates and it will still want to skid on your spring guide end (and occasionally do skid and jump and gouge on whatever it is sitting on) so you want to control these movements

 

it won't all get transferred to the front

[EDcase]

I didn't know it had a bearing on the piston end.

Yeah I think one bearing is enough but its less efficient on the piston because of the added mass.

 

 

Edited February 10 by EDcase

[Sewdhull]

5 hours ago, Pseudotectonic said:

but when the spring compresses the whole length (every coil) rotates and it will still want to skid on your spring guide end (and occasionally do skid and jump and gouge on whatever it is sitting on) so you want to control these movements

 

it won't all get transferred to the front

The only torque on the base of the spring will be that overcoming the friction in the bearing in the piston or vice versa.

If you have 2 bearings, one with a marginally higher level of friction than the other, only one of the bearings will rotate because the 2nd bearing never gets to see enough force to rotate, it dissipates once the 1st bearing is rotating.

 

 

 

 

[Sewdhull]

6 hours ago, Pseudotectonic said:

Agree you need the bearing for consistency and reduce chance of the spring getting hiccups

 

For the 18 AWG Perun wires I actually appreciate they put this in, this is some very high wire count with silicone jacket so it's very flexible which is great for working with, and I've calculated the difference of ohm between 18 and 16 AWG for the lengths in airsoft is really negligible

 

Have you tried polishing the piston groove walls on the gearbox and where the tappet plate and selector plate slides on? I touched up mine with some 5000 or 7000 (forgot which one) papers and it does make things glide on the gearbox better (now I actually have a problem with an overpowered m90 spring shooing 355 FPS because it is too efficient but that is another topic)

The currents in aegs aren't usually very high so the perun is fine.

I haven't done anything to the gearbox grooves other than check the piston slides properly. Perhaps I should.

I'm trying to be mechanically sympathetic to the gearbox and using the lowest spring stiffness I can.

[Sewdhull]

Ok, so I went with M105 spring from xt. Very consistent at 1.18 and 1.19J on .25 with hop off.

I could only find .4g bbs but they gave me 1.17 and 1.18J.

I still have to hop it so this is the peak energy and hop will scrub some off.

[Sewdhull]

I'm not convinced it's feeding quite right, it seems to but the hop rubber pokes back further than I think it should

Edited February 21 by Sewdhull

[Sewdhull]

Numbers, 1 to 4 are semi.

 

5 to 7 3rd burst, or maybe the 1st 3 are burst ...

 

 

[Lozart]

16 hours ago, Sewdhull said:

I'm not convinced it's feeding quite right, it seems to but the hop rubber pokes back further than I think it should

 

 

Maple Leaf?