12:1 Short Stroke Issues

[Starbuck]

Hi there, 

 

Never having tech’d a gun before I decided to jump balls deep into a fire filled hell. I am very proficient with tools and building machines so technical know how is good. 

 

I bought a DE M904E to serve as my teaching platform. V2 gearbox, high fps, easy to break down, built in mosfet etc. Before anyone says it I fully realise this is the most overly upgraded £110 pound gun 😂
 

I did this solely to learn how to tech through advice of a friend of who is a legendary tech and watching untold amounts of YouTube videos. 
 

So I decided to go with the following:

Gearbox:

 

Rocket 12:1 Gears 

Element Silent Piston and Cylinder Head (also have a Maxx piston and cylinder head here) 

Maxx Titanium adjustable nozzle

SHS Polycarbonate Piston with full metal rack 14 teeth

ASG 8mm Bearings 

Rocket High Torque Motor

Perun V2 Optical

Various Rocket or Guarder Springs

 

Barrel & Hop:

 

Madbull 6.01 230mm

ProWin Hop Unit (ditched and reverted back to stock rotary hop unit)

Various selection of hop rubbers and nubs. Found the Prometheus Blue Flat Hop bucking and nub to be best. 
 

Once I completed the build the issue I was having was unbelievable overspin and a whopping FPS of 160 on 0.25s...

 

So I short stroked the gears, two off the pickup, one off the release and took three teeth off the piston rack to match. Enabled AB on the mosfet and voila a better but still shocking 270ish FPS but no overspin on 0.25s. 
 

This was running a Guarder SP120 so even with three teeth off, and I may be wrong but this is was lower than it should be. I’m trying to hit 350-360 on .2s so 1.2j all in. This should be hitting 320fps with 0.25s. 

 

I don’t know where to look for the loss. 
 

The piston/cylinder/nozzle compression is excellent. The stock tappet plate spring is strong as balls. I bought a spare tappet plate spring and it was longer and weaker. 
 

The only thing I can think of is trimming the tappet plate to release earlier and maybe removing a coil or two off the tappet spring. Other than that I’m out. 
 

What do you guys think? Any other ideas where I should be looking?

 

Any help is appreciated. 

 

 

[Sitting Duck]

How do you enable/disable AB on the 904G ???

I didn't think you could disable it - it was on by default like G&G ETU & others

EDIT - you changed the 904G out for a Perun ???

 

FFS - why buy the 904G & completely gut it

why not buy almost any M4 ???

 

You likely got overspin coz optical sensor didn't pick up the teeth or grease on sensors

Also with the optical mosfets you can't just bench test them like normal boxes...

They need to be fitted into receiver or at least cover up areas like M4 pin

as light can enter & fuck with sensors & shit confusing the module

 

You should have cut 3 teeth off the pickup side imho

(if you was removing say 4 or 5 then yeah take a tooth off release perhaps)

 

Anyway....

 

Trim the tappet fin a LITTLE bit to allow tappet/nozzle to release a tooth earlier

(on very snappy high speed builds - you should consider it anyway, say trim 3 or 4mm off)

sort of in between a full tappet fin & a DSG tappet fin perhaps

 

However - the 904G isn't a screamer out of box - even on 11.1v

What battery are using 7.4v or 11.1v ???

say 24rps on 7.4v & 37rps on 11.1v - however the 904g fancy mosfet skims some of this off

so you could be looking at 20 & 30rps aprox

 

Now you used a SHS piston...

Did you check this for binding - if it falls easily in the box ???

coz these pistons - though very good do tend to bind/drag on numerous boxes...

JG, Cyma's, APS...

 

lightened SHS stuck, not falling in a APS box, they can bind

especially more so at the back of box...

 

 

your piston could be binding , slowing its return, losing power & increasing risk of PME

SHS's don't bind in G&G boxes - but unsure if they bind in DE boxes without a little mod

 

Has piston got bearing/spacer inside or not ???

any spacing inside or no spacing alters the spring's compression power

each tooth is 3mm pitch, so bearings = aprox 5mm or 1.5 teeth compression on retraction

 

if you take a m120, SS 3 teeth = aprox 45 fps drop (each tooth is about 15fps ~ 16.4fps or m05)

if you remove bearing or 5mm spacer = 24fps drop

if you remove chunky 9mm alloy spacer = 45fps drop again on spring power

if you correct AoE you lose say 10~15 fps as you shorten the max stroke (depending how you correct)

 

Now offset against some of all this - is if you get nigh perfect seals for compression

so could claw back say 30fps or so...

 

the other issue if you have poor seal with nozzle/bucking lips

(I would not have changed bucking/hop stuff until you test on established old bucking/barrel first

(you change way too much in one go - it can be harder to pinpoint exact issue

or if you have multiple issues going on at once)

 

maths, suggestions & bollocks aside, you need to go back and check EVERYTHING very very closely

[Skara]

 

I also found Guarder springs to be rather soft for what they're rated at, a SP100 gave me barely 1J on the striker and a SP90 got my old ARP (180mm inner) to barely 0.65/0.7J.

Btw, SP and M ratings are done with different bb weights, M springs are supposed to give you the number (in m/s) on 0.20g bbs, so a M100 means 100 m/s on 0.2s, whereas the SP springs are measured with 0.25s, again a SP100 would give you 100 m/s on 0.25g (around 112 m/s on 0.20s). That's the theory anyway.

Meanwhile a SHS M90 spring in the striker gave me 1.2J easy (300mm inner and a cylinder volume not far from a full v2 one), the same spring also gave me 110 m/s on 0.2s in my specna 933 (3/4 cylinder, 260mm inner).

 

Most likely a combination of the two.

 

Also you mentioned that utter POS ProShit hop unit, did you keep the same unit throughout the test? Or did you go and swap it at some point?

On 08/01/2021 at 10:54, Sitting Duck said:

FFS - why buy the 904G & completely gut it

why not buy almost any M4 ???

IKR, the FCU is the main selling point of the whole line  fucking TM recoil syndrome right there

Edited January 13, 2023 by Skara

[Davegolf]

5 hours ago, Skara said:

  fucking TM recoil syndrome right there

I don’t know what you mean there 👀😁

Seeing as your goal in this venture was to learn to tech, you kinda went backwards.

 

First off use the standard tappet plate, nozzle, hop unit and bucking, this eliminates MANY variables.

Below 230mm barrel length you’re going to want to be at least one spring power rating higher than the spring is advertised as.

 

Next up as mentioned, you’re losing 15 FPS per SS tooth too.

 

Personally I haven’t used that motor, and you haven’t said what battery your on - but you don’t normally need to short stroke with a ‘torque’ motor on 12:1 ratio.

 

You say compression is excellent, is that to say the piston stops dead and rock hard a gnat’s cock past the cylinder port?

Also as @Sitting Duck said, always check the piston is free when changing from standard, and “maths, suggestions & bollocks aside, you need to go back and check EVERYTHING very very closely”

[Starbuck]

Thank you guys very much for the replies. 
 

To answer some of your questions above:

 

Battery - Nuprol 11.1 1300mAh 20C/40C

 

Compression - It’s amazing. As soon as it goes past the port it’s like a rock with the nozzle in or extended with tappet plate fixed on end of cylinder head. 
 

Hop Up Unit - The ProShit unit is garbage. It’s going back. It’s just awful. I’m using the standard rotary hop. 
 

Piston Head - The Maxx and the Element both have the bearings inside them. I’ve noticed the spring sits with the bearing inside the spring. Is this right? 
 

Mosfet - Yeah I bought the M904 because A - it was cheap and B - because it had a mosfet. The only reason I swapped it out was I thought I blew it when I plugged in a 11.1 15c 2200mAh. I’m sure it wasn’t this now and it’s actually fine. 
 

So before I got a chance to read them I stripped it down and checked the piston, it was binding on the gearbox when it went back and we are talking something awful. I had another piston there and swapped it out for that and that brought the FPS up to 310-315 on 0.25g. 
 

The two issues I’m now having is the ball flight is garbage. It wasn’t great stock but now it’s a bit ropey. Hop is all over the place and is hard as balls to set. The BB’s done flat straight they fly all over the fucking shop. 
 

The second issue the gun doesn’t work worth a fuck on burst or full auto. It’s like the fun loses all power and the drop like stones 30 feet out the barrel. I think this is possibly because I shaved too much off the tappet plate. I have a new Rocket tappet plate there I can fit and shave a little off. 
 

If you have any input on the above that would be amazing.

 

thank you again guys. 

[Sitting Duck]

list:

 

https://www.ak2m4.co.uk/internal-parts/hop-up-buckings-nubs/maple-leaf-macaron-60-aeg

&

https://www.ak2m4.co.uk/internal-parts/hop-up-buckings-nubs/omega-nub-soft-70-gear-parts

& maybe:

https://www.ak2m4.co.uk/internal-parts/hop-up-chambers/zci-rotary-hopup-chamber-plastic-m4

 

Now you are using 11.1v on 12:1 with a 28~30k motor

so estimate rps @ say 25rps on 7.4v which I advise you to try first to get shit running OK

on 11.1v you will likely be hitting 38/39 rps, though battery might only get to 35rps - still a bit too damn quick

 

build a new hop/bucking/barrel to swap in/out

 

the bearings in the piston - you used threadlock I hope as they have a habit of coming adrift

(no matter how damn tight you tighten the screw up)

it's the bearings, as they move & transmit impact/vibration it just starts to undo & crunches up in cylinder

 

bearing doesn't or shouldn't sit inside the spring as such, there is like a 15mm washer/race

with a collar holding it in place - that collar on far right sits just inside spring...

 

you would probably need to mod the tappet a bit for a snappy gun cycling quickly...

 

**** though I'd trim it not quite as much as I did - somewhere in between the two above ****

 

I'd still run it on 7.4v first to iron most issues out first,

then see if tappet needs more trimming or higher tension spring to return more quickly

the ONE tooth you took off release isn't THAT bad, compensated by slight trimming of fin from full tappet fin

 

however I don't know if the nozzle is correct length or if tappet is 101% suited...

ensure the rails are smooth inside box...

DON'T GO NUTZ WITH EMERY PAPER !!!

stuff like cleaning internals with toothbrush

& any smoothing of piston rails & tappet rails - use a really OLD kitchen scourer

(basically worn away to shit, only mildly abrasive - clean/smooth surface but not wear it away

something just to smooth the surfaces without wearing it away to bits, really knacked scourer works well)

 

 

something like that will do to clean up rails n channels n shit

(test for any burrs & then, only then lightly sand any real burrs in that exact area)

Ensure the tappet glides smoothly back n forth - all clean up front of box, no crap/shit etc...

SHS tappets are a bit chunky up front, perhaps see if the front can be sanded a little to ensure nozzle seals well

(they can be a little untrue, not quite perfect 90 degrees, drop in cup of hot water for a min or so

then straighten - carefully & should be more true 90 degree)

 

an area to watch for is the tappet fin hitting the sector axle that can impede it returning fully

some axles are more chunky than others & might need to use a needle file to CAREFULLY file the EXACT area on fin

so it no longer hits/impedes the tappet returning 101%

 

 

as the std Cyma one had a chunky bit cut out to miss the axle...

 

 

that is the stock CYMA fin untouched btw

(they have extra material inside box hence the modding of SHS tappet up front)

 

all this shit is something you have to investigate after learning what a cnut these toy guns can be

 

Other stuff like your sector could be shimmed a bit high and possibly rubbing on tappet plate

that can rub/impede its effective operation...

(see if the unsprung tappet slips back n forth without moving sector gear (bevel/step removed)

or rotate sector through cylinder area & see if it drags the tappet back with it if rubbing

 

basically you have to double/triple check EVERYTHING as various bits n bobs do NOT work perfectly

the TM Compatible bollox is more like TM Compatible-ish at the best of times

some shit works OK & other stuff is only gonna make matters worse

 

hence numerous bits may need a minor mod

every part MUST be checked & compared with the old part(s) it replaces very very closely

often check sector/tappet/cylinder operation in top half of box

(saves messing with wiring in lower half but less play/wobble in top half of box when checking how it all glides)

 

 

 

with delayer BUT WATCH FOR TOO MUCH RETRACTION or bottoming out on cylinder head...

 

 

WAY WAY TOO MUCH INFO STUFF - but more to show the checking of shit in top half of box

it is all loads & loads of checking shit - mostly not needed as it will all pass the checks OK

 

but if you change LOTS like you have you really need to check LOADS of things - well EVERYTHING

so it all operates perfectly and smoothly everywhere

 

There is other stuff like if you tighten the stock M5 bolt up TOO MUCH it can pull the box back

and in turn tilt the box upwards at front slightly, which when tilted up can rub on hop unit as it enters back/forth

Normally this happens on metal receivers than plastic ones

as plastic receivers do allow a bit of flex so it doesn't pull the box back/up

 

The nozzle "shoulder" shouldn't be too snug fit into the hop unit

the red SHS nozzle for example is a bit snug entering brand new G&G hop units

so can impede the whole shit bollox operation of chambering & sealing effectively

(if using another hop or worn in G&G hop unit the nozzle moves freely back n forth at speed everytime)

 

again might not apply to your gun, but just some examples of where & how these poxy guns can be such cnuts

[Starbuck]

Thanks for the reply 👍

 

My plan now is the following:

 

1: Use the Rocket tappet plate and sand it down a little as I think I removed too much from the standard one so it was returning too fast on full auto. Semi loads fine it’s just burst or full auto it struggles. Struggles a little if I spam semi auto as well. 
 

2: I need to try and clean the barrel and hop rubber. I never cleaned the barrel once since it arrived and what I gather they all need it. Insane. I can’t imagine putting any of my previous machining work out not immaculate. Different standards I suppose. This could be contributing to the random ball flight. 
 

The ZCI hop unit you linked me to looks identical to the one in the gun. I’ll get it for the few quid it is and also the hop rubber and nub to recommended. Going to get a 5” barrel extension on it and go to a 363mm ZCI barrel. 
 

 

[Starbuck]

OK guys quick update. 

 

I installed the Rocket Tappet plate (SHS), taking care to see when it was fully back, sanding a little, then checking the release and sanding the lower fin a little. I also shortened the tappet spring by 2 coils.

 

Gun now shoots lasers bang on legal limit on semi...

 

Auto, it drops 40 fps and is a bit inconsistent. 

 

Im going to order a 7.4v Lipo and try it on that and see if full auto works more consistently. 

 

I really appreciate all the help guys. Thank you. 

[1337Sinned]

Im building a high speed ssg setup myself. It is based around the ultra high speed warhead. 

 

So it was interesting for me to follow this thread. Thx for all info.

 

One thing that stood out by absense is air leak on bucking. It was mentioned but a really good fix was not there. There is the dental flos fix and the teflon tape fix. Both are a bit sketchy cos it doesnt last. 

 

What I did to fix leak from bucking is I superglued the last 3 mm of the bucking to the barrel (The side where the bb exits). 

 

The bucking lasts long enough to switch out with a New one whenever i need it removed and the glue comes of the barrel very easy.

 

Normally I shot 1.7j with 0.30gr and 1.12j with 0.40. Now I shoot (had to rejust the adjustable springguide) 1.7j 0.30 and 1.63j with 0.40gr.

The 0.07 loss is imo escaping from the nozzle between bucking or cylinder head. 

 

It a bit of a spartan method but I can recommend it highly 

[Pollynator_bravo2]

From my experience if it’s constantly coming in low but around the same value in terms of fps I’d look at the air seal and timing. If it’s inconsistent I’d look towards tappet sticking or the nozzle not consistently travelling parallel with the hop