Cyma Cm.621 Upgrades

[Misery]

Well my cyma's piston has crapped itself,any recomendations on a upgrade piston? also thinking of replacing the gears shimming etc as im assuming the issue has been the gears been able to move and jump cause it to do that,i dont mind throwing a little bit of money at it as i dont have much invested in it so have a rough budget of bout £70 for upgrades/replacement parts once i have the mechanics all sorted will move onto cosmetics

View attachment 67672

 

[Sitting Duck]

That is "likely" PME Premature Engagement

gun cycling too fast, sector attempts to engage the piston before it has fully returned

and crunches into the rack on the 3rd/4th tooth etc...

Plastic teeth being weaker start to strip, it happens once or twice, it gets worse = f*cked

If this happens on a full metal rack piston then metal gears/rack tear themselves off

(something has to give/fail in the end)

It can also happen with a jammed BB, so the newbie mistake is to keep firing on auto to clear it

even mag out - full auto - CRUNCH

The jammed BB has plugged the compression like a syringe and this slows the piston's return = PME

or something just failed, pear shape tits up and crunch - stuff happens

but usually that looks like possible PME likely cycling too quick or jammed BB or both

Change piston to metal rack Cyma, buy new o-ring

could buy new cylinder head, but 7 or 8 wraps of ptfe works well if done neatly

leave the gears, maybe buy a SHS HT motor but NOT a high speed motor

that'll do for internals

clean barrel, use decent bb's - really makes a difference & prevents jams from egg shaped bb's

(or big seams on cheapo bb's causing jams in tbb/bucking)

if you want zci hop, maple leaf bucking & red nub

barrel should be a half decent tight bore one

 

[Misery]

was trying to figure out what you meant by pme loool

perfect and again thank you for your time replying

as far as i can tell it has had a hop rubber changed to a maple leaf and i think its got the red nub installed as theres an near empty pack with just the black one

View attachment 67675

 

[Sitting Duck]

Ahh - good stuff

the previous owner bought the twin pack nub hard black/soft red

you need to check it was fitted OK by plopping a bb into hop unit

then it should only need a light moderate push with match/cocktail stick

or very carefully a thin screwdriver, just something to pass the bb through lips

if it is tight then that will misfeed or mischamber the bb causing misfeed/jams...

somewhere in this vid he goes through the "push test"

DON'T GO MODDING SHIT !!!

it is just to show tight bucking lips causing feed issues

tbh just test the lips are not too tight - that's all

I doubt if you got tight bucking, but always worth a check coz you will have issues feeding

again do not do anything like modding hop etc...

just check the lips are not too tight on bb push test - that's all

 

[Misery]

will give that a check in the morning thank you

yeah i wouldnt go modding things without posting here first bud i dont know enough yet to even contemplate that and only got the basic theory of how the hop works havent delved in to that side further yet lol

 

[Adolf Hamster]

Going from your other thread you're running that thing on 11.1v

In which case i reckon @Sitting Duck is spot on about the pme but i'm wondering if this is a weak spring as the culprit.

ive seen a cyma eat a piston like that was fresh out the box, 11.1v but a half-assed "fps downgrade" by the shop with it spitting out 0.5j had the piston chewed up bad within 30 minutes (weak spring means it doesnt have to be spinning as fast for pme to be an issue so the stock motor on a hefty battery was fast enough)

When you get the new piston in might be worth checking the fps. If you get very consistent (ie each shot doesnt change much in fps) but low readings then that might be your culprit.

Dont know how confident you are with tinkering but there's a few tips and tricks for dissassembly/gearbox checks here:



You dont need to go ham with trying to change things like aoe/shimming etc just yet, Just keep the shims in the positions they were originally. but at least that should let you know what to expect so you dont end up with a spring guide pinging its way into the aether the moment you pop it open.

Of course this is assuming the gearbox is all stock and this isnt as simple as the previous owner dropping a spicy motor in there.

 

[Sitting Duck]

Going from your other thread you're running that thing on 11.1v

In which case i reckon @Sitting Duck is spot on about the pme but i'm wondering if this is a weak spring as the culprit.

ive seen a cyma eat a piston like that was fresh out the box, 11.1v but a half-assed "fps downgrade" by the shop with it spitting out 0.5j had the piston chewed up bad within 30 minutes (weak spring means it doesnt have to be spinning as fast for pme to be an issue so the stock motor on a hefty battery was fast enough)

When you get the new piston in might be worth checking the fps. If you get very consistent (ie each shot doesnt change much in fps) but low readings then that might be your culprit.

Dont know how confident you are with tinkering but there's a few tips and tricks for dissassembly/gearbox checks here:



You dont need to go ham with trying to change things like aoe/shimming etc just yet, Just keep the shims in the positions they were originally. but at least that should let you know what to expect so you dont end up with a spring guide pinging its way into the aether the moment you pop it open.

Of course this is assuming the gearbox is all stock and this isnt as simple as the previous owner dropping a spicy motor in there.

luckily....

he has the QC Cyma box with mosfet in there

which helps to open up with spring removed

(ahhh the fun explosion I had opening my first SRC box)

it is a weird non-reinforced at back Cyma - 13:1's will go straight in

(ergh - keep gears as is btw, don't change more than you need to)

A SHS piston "might" not bind like the reinforced Cyma boxes

(both v2 & the .028 v3's - SHS pistons bind in those)

Still just replace with Cyma piston is safe option imho & reassemble

Blue tappet is way better than old clear tappet

Cylinder head is same floppy fit - only in blue than white

Piston head - sometimes a silent mushroom type

other times it is same concave piston head now in blue than white

(still needs o-ring stretched/changed for compression)

Nozzle is same std type non-o-ring but in blue than clear

got a 517 in November - blue stuff inside

8mm cross bushings "seem" to be slightly different/revised

or new source, tiny bit chunkier, slightly less shimming room

than older original 8mm cross bushings I remember

box was still reinforced at back, mod req for 13:1's

has a ARL window in it now it didn't have before

Std trigger switch - OMG this is blue and the shuttle/trolley

has loads of lateral play in it, I slightly pinched the contacts straight (less flared)

shaped them to touch but not grab the trolley...

Fucking thing was moving sideways all over the place missing the contacts at times

Don't remember the old white trolley switch as loose as this

So changed the switch/trolley due to lots of lateral play in it

The sloppy blue switch in the new revised cheap 517/5xx M4's is my only gripe

(and still uses the prongs at selector plate)

need to often carefully widen the switch mounting "eyelet" if using other v2 switches

Still got the silly safety arm to undo prior to opening box

But overall pretty robust stuff inside with mostly minor improvements

(still crap o-ring & loose as fuck cylinder head)

Not a fantastic improvement, nor say another half a point

but say an cheap as fuck 8/10 became say a 8.25/10

 

[Adolf Hamster]

after looking at this thread without being sleep deprived i realised how has he photo'd the piston if he hasn't already taken it apart....

 

[Sitting Duck]

Them Cyma QC boxes are something else

when the piston shits the bed they eject the piston for you

nah - get some more kip

he said he opened it up to check what was fucked here:





click arrowy bit to see the reply...

so decided to crack the gearbox open and take a look myself

as to why it shit the bed on just 5 rnds on semi ???

dunno if the Cyma bespoke mosfet is an AB type

or just a std mosfet - really haven't a fucking clue

could be std, could be AB with the extra bit of pcb bollox

if AB then it should have made the piston stop on a tanner after cycle

& not PME like a std mosfet could

but reckon it was a jam on nutty auto burst that likely killed it

or seriously injured it at some point

(most PME wrecks occur on auto - well mine have when taking the piss

& even with planning - still wrecked on auto seeing oooh this goes quick - oh FUCK)

 

[Adolf Hamster]

look i have the ability to self-jetlag under normal circumstances and lockdown aint helping

 

[Sitting Duck]

look i have the ability to self-jetlag under normal circumstances and lockdown aint helping

I'm wide awake most times but still veer right off the actual subject

so far as I forget what the bloody original question was...

sorry what was I saying ???

Ya probably right 11.1v used causing rapid shredding

guessing these issues is a fun challenge at best of times

I love it when the poster just mentions they was running it on 14.8v LiPo

and you then think - oh just fuck off now

or - yeah it might be a short as sparks were flying out the back blowing the fuse

so I just used silver foil to bypass it - WTF ???

or yeah it did make a big crunch, jammed up but now it is working but nozzle doesn't move much

you know the 3 pages in & then you start getting the rest of info how shit broke

not directed at OP, has been fairly up front - just sometimes it is a major guessing game

THEN the real truth comes out - wtf didn't you say that at beginning ffs !!!

Sorry what was question again - I'm waffling again

 

[Davegolf]

@miserydrift I would fit a similar plastic / metal teeth combo piston, BECAUSE if shit hits the fan, like it has for you, the damage is limited to just a new piston.

If you run full metal teeth piston and what has happened to you happens again, you can break SO MUCH MORE!

Soz @Sitting Duck ?

On a budget get this piston and spring;

https://www.ak2m4.co.uk/internal-parts/pistons/rocket-3-tooth-piston
 

https://www.ak2m4.co.uk/internal-parts/springs/aols-aeg-spring-m100

If you want a stronger piston that is still a great fail safe, then get one of these;

https://www.fire-support.co.uk/product/ics-reinforced-upgrade-pom-piston-set

 

[Sitting Duck]

 @miserydrift has hit his daily post limit it seems

so this is why he hasn't responded it seems

@Davegolf - yes metal rack pistons give you weak point

but the SHS 3 steel & 7 steel pistons are shit imho

the plastic is softer not so durable like stock Cyma/JG 1 metal tooth pistons

in fact SHS 3 or 7 tooth pistons are weaker than G&G stock pistons I've found

the plastic teeth stripped quickly - way too quickly and at mid 20's so not PME

plus SHS's - well the metal rack ones may bind in there so expect the 3/7 teeth ones too

(no biggy but a bit of modding will fix)

however I just think the SHS part metal rack pistons...

well the metal teeth are fine it is the plastic teeth is just a bit DairyLea imho

weaker than decent stock pistons like Cyma/JG as I said

so the OP now might have an understanding on keeping the ROF down a bit

so a metal rack will be fine & durable...

AS LONG AS THEY DON'T GO NUTZ - which hopefully he understands it a bit better

OP might grasp the fact there is no fail-safe or weak point to avoid risking PME

if push came to shove I could send him some stock Cyma pistons

(hardly been fired), if he was that worried

but as long as he understands the limitations/rof stuff he should fine with a robust full metal rack

yes maybe part metal rack - but nope not SHS 3 or 7 tooth rack

(Sorry SHS but your plastic is way too weak/soft)

people can argue or differ in their opinion on these pistons but had one fail very very quickly on me

also - remove the 2nd tooth etc.. on a SHS part metal rack - see how easy it is to remove with a sharp knife

cor - this is easy...

yeah it fucking strips easy too in use

then remove a tooth on a hard Cyma/JG piston - not quite so easy coz it's more durable material

fucking shame coz SHS pistons are nice n light - just really don't think they are that durable tbh

(sorry if people think otherwise - but I gathered up any I had and flogged them on fleabay years ago)

 

[Davegolf]

I completely agree, I don’t think they are THAT durable, but good enough for a stock ish 1J gun.

Any POM half rack all day for me.

 

[Adolf Hamster]

kind of agree on shs's plastic, assuming it's the same as the bodies of their full rack pistons because that's what i'm normally looking at running.

if the gun is put together right and not run too hard then either works, but plastic does offer a bit more insurance for the gears if you're worried about it being too fast.

either way it's a pain in the ass to clean up when one goes

 

[Sitting Duck]

Seems like the hop might have caused a BB to jam than actual speed

which resulted im PME piston shredding

he can't post atm so is posting in updates bit

maybe lips or hop on way too much - well what ever

Another piston to be careful is the 3 & 7 tooth dark grey ones from TWG

they are a bit shit also

had one just rip the rip initial pickup tooth right out

https://www.taiwangun.com/piston/reinforced-piston-with-seven-steel-teeth?from=listing&campaign-id=19

Hard as fuck - like nails that plastic, like brittle hard as fuck plastic

had some 3 & 7 tooth versions...

I think the pick up tooth is way too large/deep and kicks the sector as it returns

had one of those just crack the pickup right out

was a long time ago - but knowing what I know now

I could carefully file the massive pickup down a smidge

and keep checking shit in top half of box as I often do

yet my gut feeling is just a tad ultra hard & brittle plastic/polymer

but another piston I tried and thought nah fuck it

I got a few pink OEM ACM 7 tooth pistons - they're fucking ace

as well as some pink full metal rack pistons about 3 years ago from GF

for a few quid - they were fucking awesome & didn't bind in boxes like SHS did

wish I could find them but reckon they are long gone from production

would be like finding a fixed stubby M4 stock - like rocking horse shit or hens teeth

nah - tried 3 part metal/plastic pistons and only one of them was any good

and that one is no longer in production it seems - just my luck

so tend to stick with metal racks myself - but to each their own

here are the OEM ACM pistons that I love...

the 50/50 rack is at back - shiny surface

but these are damn good pistons - that don't bind like SHS's

need a bit of work/cleaning up but fits the bill when a SHS binds

if anybody sees these around they are pretty good pistons imho

(probably seen them in some of my build posts)

I'll use a SHS metal rack in a G&G as it don't bind

Try a Cyma metal rack in others

but if the G&G is too tight, the Cyma is a tad loose

(bit too much plat left/right & up/down as piston moves back/forth)

Then one of these pistons are usually chosen as they are in between

a tight SHS & sometimes smidge too loose Cyma 

yup they are/were damn good 'uns

where as the SHS & TWG grey 50/50 pistons - not such a wise investment

Something similar to those pink 50/50 pistons is this...

https://www.taiwangun.com/piston/reinforced-piston-with-seven-steel-teeth-bw?from=listing&campaign-id=19

Often I didn't need them or out of stock to add to a previous order

BUT if anybody is doing a TWG order at some point & might need a 50/50 piston

well I reckon they could be worth a gamble on buying a couple of them

That was about the only piston I saw like my 50/50 ones I've used/rated

hasn't got the full complete intact rails but nigh on

might need to check the initial pick up tooth isn't too high

(in some boxes rubbing on sector's toothless area, very fine file)

extra material on 2nd/3rd tooth surrounding area - bit of work removing

chunky pickup tooth, won't need much to even partial correct AoE etc...

(yes often shit needs a bit of tweaking, glue the rack etc...)

if it is like my pink ones, they shouldn't bind in most boxes

but not too much play/slack either - just the right fit to glide without binding

End of the day, it's a case of checking & getting whatever shit you got to hand

all operating smoothly & efficiently etc...

 

[Adolf Hamster]

ahh yes, see the status update. so my spring theory was wrong.

you win this time duck

 

[Sitting Duck]

Didn't win anything - just guesswork

but we've all broke shit lots of times in many different ways

THIS is my favourite of favourite pic that sums up

diagnosing shitty bastid toy gun issues....

poor OP won a fucked piston & lost his posting ability

whilst having to wade through all my crap instead - ouch that's gotta hurt

 

[Davegolf]

Yes this is most common fail, basically pneumatic lock

“Seems like the hop might have caused a BB to jam than actual speed

which resulted im PME piston shredding”

One of the reasons 6.00 / 6.01 barrels are daft too. Dirty jam.

 

[Adolf Hamster]

Didn't win anything - just guesswork

but we've all broke shit lots of times in many different ways

THIS is my favourite of favourite pic that sums up

diagnosing shitty bastid toy gun issues....

personally i prefer "the master has fucked up more times than the apprentice has even tried"

poor OP won a fucked piston & lost his posting ability

whilst having to wade through all my crap instead - ouch that's gotta hurt

i still think one of these days we just need to go through your post history and stitch together a proper "how to gearbox" guide

One of the reasons 6.00 / 6.01 barrels are daft too. Dirty jam.

yeah, you do have to stay on top of cleaning with the tighter bores, but then regular cleaning is still the best cost/performance ratio "upgrade" for any gun.