A Thread About Skara Questioning His Arp-9

[Skara]

New bushings in, tried spur first and it's dumping a moulinex into a bucket of nails and a spanner levels of scratchy.

Tried bevel to pinion, still scratchy but without the spanner.

Also tried with the stock motor, still the same. Good thing about these Modify bushings is that they have a little ridge protruding into the gearbox so I didn't use too many shims.

 

At this point it's pretty clear, I got a lemon somewhere.

I ran the thing for a couple of minutes to try and scratch the gears, the shell or anything else and locate the source. Gears are spotless, the shell has just a tiny bit of scratching under the spur (fixed by swapping a 0.15 shim with a 0.2).

Bevel teeth seems fine, pinion seems fine (I might try and swap it for a SHS one but they look identical).

With the motor inserted and the height somewhat adjusted there is just a tiny bit of play on the bevel, with the shell closed and tightened each gear has about 0.1mm of play which looks bang on to me.

There is no grease that may conceal scratches.

I'm giving a SHS HT a go, just to eliminate the motor as a cause.

 

I'm legit clueless, anyone got an idea?

@Sitting Duck

[Adolf Hamster]

tried it with just the gears in the shell spinning by hand?

 

also worth checking backlash, even a completely matched set of gears can sound bad if the box has them spaced slightly too far apart.

 

other thing worth checking is the idler in particular can commonly not be flat, so whilst it might spin freely for part of its cycle it could bind at a single point.

[Sitting Duck]

Motor grip/angle is my hunch

 

but tbh it could be anything, wire either side of motor

loads of slack in the gearbox opening & motor tower

so it thrashes about...

 

Motor grip OK on gearbox out of receiver ???

so the grip needs filing as it could be pulling when all fitted together to receiver ???

tried marking the box with receiver & also grip to see if grip is too large ???

 

something like the motor axle is not 101% inline with bevel axle,

so it is running off centre or at a minor angle if motor slightly skewed

 

I'd grab another grip if you got one, one that is pretty snug on box

and doesn't have a massive motor entry point to the box

so hopefully the motor is not able to thrash about once it all aligns

(hopefully in sync & in line from motor axle shaft direct to the bevel axle

so the bevel pinion is exactly at 6 o'clock & not offset)

[Skara]

I swapped motors with my 933.

 

ZCI motor in the 933 sounds perfectly fine although it wants to sit 1/1.5mm lower than the SHS.

 

SHS in the PDW (just the gearbox with ICS pistol grip) sounds shit, no matter how high/low I set it.

 

Spinning gears by hand you can definitely hear them grinding, I also had doubts about the spur/idler not being flat because when I had the box shimmed spur first, at one point it was getting stuck with the bevel. Now it spins fine though (read scratching but not binding).

 

So the motor is ruled out, the gears might be part of the issue and I can still replace them with ZCI advanced (my tech has some in stock, a lot more expensive than buying online though).

The major suspect remains the grip then.

Unfortunately, other than the ICS mentioned (which is also wonky as heck) I don't have any spare.

I was thinking of getting a couple of PTS EPGs for my M4s sometime soon (tm) so I could give a specna A2 a go.

 

Can't be arsed right now but I might try tomorrow.

[Sitting Duck]

Be careful of them Magpul/PTS clone grip bollox

I had a couple and some other aftermarket grips - they were shite

motor sloppy as fuck, motor grip plate seem to shove motor way out of line with axle

certain motor plates don't always fit other grips - despite looking the exact same

 

So come to the conclusion that most often the stock grip is kept as it usually is fair good

(sod changing grips to a more fancy looking one)

 

Sounds like you might have a burr or slightly wonky gear or two also

adding to stuff - sometimes a rebuild goes according to plan, almost completely flawless easy shimming

other times it is nightmare after nightmare and have to go back with an ultra fine examination of everything

 

Yes I fully understand how some motors, the way the armature is set on the motor's axle higher/lower

results in some motors being nigh on the max/min limit in some guns/grips etc...

also the springs on the motor tower can vary too, usually they compress right down to say just under 5mm

(squishing them & measuring in a digital caliper)

but others only compress down to say 6mm - so before you start the motor can be sitting 1mm higher

due to a motor spring with more coils in it

 

Then factor in the way armatures sit higher/lower on some motors and in middle of magnetic field

(like SHS & ZCI)

then no wonder swapping motors can result in a very different motor height setting

and sometimes this can be beyond the tolerance of motor height adjustment on some guns/grips

 

fucking nightmare it can be - I know some will be tempted to add some shims between grip/box

to space the grip if motor sits way too high - well as temp fix or to see it if improves

(I guess if you had 4 screw threads in box you could add 2 to left/right to slightly tilt motor angle)

but really this is a temp fix/test or bodge trying to see if motor aligns a little better

but really try another grip/motor/pinion/gear-set or something and go back over it all again

 

end of the day you know by the sound if it is right or not, backed up by amp draw

but you know when it sounds right - it's your own fault for being a fusspot when dealing

with lapse Chinese tolerances is poxy toy guns

[Skara]

So, smol update.

Since I have bought them PTS (genuine ones, not clones since they were just as expensive lol) grips I transferred one of the stock specna A2s on this gearbox..

ZCI motor still sits too high, SHS motor sits fine, can be adjusted, but the box is grindy as fuck (most likely because the bevel is super high, or because the gears are fucked or something is wrong with this new box).

 

At this point I will proceed in this order:

1) reshim (spur first) to accept a SHS HT and use the zci in one of the specnas, probably the 933;

2) revert back to the stock KA gearbox and keep this 8mm one as a spare.

 

Shame, as I thought it would work

[Skara]

I think it's the bevel...

Can I swap it with a different one or will it fuck the ratio?

[Adolf Hamster]

4 minutes ago, Skara said:

I think it's the bevel...

Can I swap it with a different one or will it fuck the ratio?

 

you can, but you run the risk of issues such as backlash or different teeth profiles.

 

as long as you're checking for those it's all good. i've done mis-matched builds in the past this way.

[Skara]

1 hour ago, Adolf Hamster said:

 

you can, but you run the risk of issues such as backlash or different teeth profiles.

 

as long as you're checking for those it's all good. i've done mis-matched builds in the past this way.

Nevermind, there was some crap stuck in a bearing

 

I've officially reverted back to the stock KA gearbox, whenever my PTS grips arrive I'll test the ZCI motor in my specnas and use the SHS HT in this one with the stock grip.

I might even get a game day with the thing before retiring

[Skara]

I give up.

 

Shimmed for the SHS motor and A2 grip, put the gun back together and installed the zci motor by mistake.

Fucking bang on (the adjustment screw is just a smidge from rotating freely).

 

What the fuck.

 

 

 

Aight, imma keep it this way, in the meantime I managed to get my ring finger caught by the piston while testing (don't ask how, because I legit have no clue) so now the gearbox is lubricated with blood

 

This gun is cursed, it's probably going to kill me in my sleep someday..

[Adolf Hamster]

20 minutes ago, Skara said:

Aight, imma keep it this way, in the meantime I managed to get my ring finger caught by the piston while testing (don't ask how, because I legit have no clue) so now the gearbox is lubricated with blood

 

how?

[Skara]

11 hours ago, Adolf Hamster said:

 

how?

 

Finger, this is piston.

Piston, this is finger

*now kiss*

 

Pretty much what happened, the crime scene was the rear cutout section of the box.

[Adolf Hamster]

that is definately a new one.

 

been stabbed plenty of times by springs and various pokey bits but never a piston......

[Skara]

Fucking bang on.

Ran flawlessly throughout the day.

Very pleased with it, just need to test if it can lift 0.32s.

[Adolf Hamster]

17 minutes ago, Skara said:

Fucking bang on.

Ran flawlessly throughout the day.

Very pleased with it, just need to test if it can lift 0.32s.

 

i'm assuming you're running a macaron or equivalent so the answer is i'd be surprised if it didnt.

[Skara]

15 hours ago, Adolf Hamster said:

 

i'm assuming you're running a macaron or equivalent so the answer is i'd be surprised if it didnt.

Yep, I'm running a MR 50° but I didn't phrase it right. What I meant is: will it hop 0.32s at 24 rps without acting funny?

 

I also need some high caps (because reloading every once in a while is fun), but from what I've seen mp5 high caps are a bit shit.

[Adolf Hamster]

2 hours ago, Skara said:

Yep, I'm running a MR 50° but I didn't phrase it right. What I meant is: will it hop 0.32s at 24 rps without acting funny?

 

assuming it feeds, can't think why it wouldn't if it's running ok on lighter.

[Skara]

On 04/03/2021 at 21:48, Skara said:

This gun is cursed

This aged like fine wine.

 

Receivers (yes, both) are badly bent, I knew there was something off but didn't pay too much attention until yesterday night.

Upper is not compatible with other brands as it's 3/4mm longer.

Buying a replacement from King Arms is ludicrously expensive because of postage and import fees (€70+, not even counting the price of the receiver set itself) which makes it cheaper to just buy a new gun.

 

 

At this point there is a whole new gun I'm looking at....

Stay tuned for more OCD autism

 

[Skara]

So, this new gun.

 

magazine wise it doesn't like stock high caps, there is a bit of wobble and I have to move them around to "feed" 5/6 BBs before needing to wind them again.

Absolutely flawless on the G&G ARP-9 drum, it sits very tight (actually needs a tap to lock in place) but feeds every BB, specna midcaps are okay although the one I have seems to have a too weak retention tooth, first time I loaded it some bbs went flying, feeds fine.

 

I have tried the bevel to pinion shimming method on this one, unfortunately the amp meter I have doesn't want to register semi auto, spamming the trigger showed 20 amps, while full auto showed 20 at startup, then 12 still going down.

This on a 11.1v 1450mAh 60c.

Energy and rof are 1.25J and 27 rps. Cycling in semi is quick.

 

Setup is:

RA gearbox

Perun Hybrid

Wanker Maxx adjustable trigger

Zci 22tpa

Rocket 13:1 gears SS'd by two at the front

SHS m110 (m120 really)

Type B cylinder (for 200-250mm barrels)

Knackered shs nozzle, to be replaced with an FPS one

Maxx TE hop unit

ML 70° Super Macaron

Prommy 6.05x208 barrel

 

Range/consistency wise I've only briefly tested it yesterday night on 0.25s, range seems okay but there is a weird horizontal spread that I will have to address somehow.

[Skara]

'nother update.

 

I'm turning this into a DSG, for maximum wanker factor and because I'm bored.

 

I have the following parts coming:

Shs dsg sector + tappet plate

Modify spring spacer

Retro arms dsg piston body.

 

I'm keeping everything else intact, hopefully I can make it work.

[Skara]

Alright.

 

First test shows 37.3 RPS, 85 m/s on semi but 60 in full auto @11.1v, which shows that:

a) it feeds on the g&g drum;

b) I have to adjust the tappet timing because the results indicate that the piston is being released when the nozzle is still moving forward, I will cut the fin half a millimetre shorter, shorten the tappet spring and, just for testing purposes, slightly modify the tappet pickup point to have it delayed by a tiiiiiiiny bit, just to rule out tappet PME.

[Skara]

Test no.2

 

Shortening 4 coils (two on each side) of the tappet spring alleviated some of the power loss issue, along with a slightly stiffer spring (shs m110 + a spring guide spacer) and a smidge more air volume by running a different cylinder the gun now does 94-96 m/s on semi and full auto on a 7.4v (20 RPS), but still drops to 92/93 on a 11.1v at 37 RPS.

The following animation shows what I believe to be happening inside the gearbox, nozzle starts to retract while the piston is still moving....

So I'm thinking, what if I knock off a tooth off the release side(s)? This way the piston releases a tiny bit earlier which in turn gives it more time to hit the cylinder head before the nozzle is pulled back.

 

Thoughts?

[Skara]

My thinking was correct.

 

Knocked off one tooth off the release side(s), BB velocity is now consistent between semi and full @39 RPS.

Also found a random spring in my box o' bits that upped the energy to 1.05J before short stroking, now it's sitting at a very comfy 0.96J (98 m/s on 0.2s).