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Mk18 Overhaul - Bit of a Fail


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I was pretty excited to break down my MK18 mod 0 for the first time in a few years, service it and install some new bits n bobs. Did the same a couple of weeks ago to the MP5, while that went flawlessly, this did not.

 

This is more of an after action report than a request for specific bits of help, however feel free to chime in with any suggestions.

 

Out with the old, in with the new:

 

- Basic ACM 8mm 'SB' v2 shell out - Classic army 9mm QD shell in (possibly out of a scarab or nemesis)

- SHS 8mm bearing bushings out - SHS 9mm bearings in 

- Old CA M15 gears out - ZCI advanced bearing gears in

- Old bog standard 3/4 cylinder out - possibly a nickel plated one in - hole location somewhere between the 3/4 and 1/2 (on paper better suited to a 250mm inner barrel)

- Old CA yellow poly-carbonate piston and piston head out (a bit shredded at this point) - replaced with a decent looking nylon one I had in my box of spares.

- Element single O-ring cylinder head out - un-branded aluminum double O-ring Cylinder head in.

- Tappet plate and spring also replaced - quite a bit of wear on both tbh (smushed back of tappet plate where it contacts the sector and the spring just felt a little too soft).

- My own flat hop out - ML Macaron 60 in (might swap this back - this mk18 is was a laser as is - why do I rip things out for the sake of ripping things out?!? dumbass)

 

First issue - the GB would not fit in the lower - quite a bit thicker overall than the old ACM shell - bit of dremel action on the lower receiver around the wear marks fixed this.

 

Reassembled, fired and the piston got stuck right at the back of the shot cycle and the motor whines horribly.

Open it up and notice there is excess plastic on the rear track both sides of the piston - clean that up, assess everything, cycle the system manually, check engagement - everything looks fine.

 

Back together in the lower with pistol grip on. test fire and everything seems pretty good - adjusted motor height got it going at a really nice crisp sweet spot.

 

Only odd thing at this point is that the shot cycling in semi auto is as follows: first shot finishes at probably 90% pre cock, next shot finishes correctly with piston all the way down the cylinder: these two starting positions repeat 100% of the time in order (mosfet is a very basic King arms no frills, AB or pre-cocking on this thing)

 

Finished reassembling, and the same again, after one or two shots, piston stuck at the rear of the track and a nasty whine - But it was okay one minute ago! - Aha! maybe the stock screw! - remove the stock tube - everything works again. So I reattach the stock but tighten the screw little by little into the back of the spring guide until I can force the same malfunction - this worked - I backed it out a little upon getting it to break and it seems to work fine - however the screw is already walking it's way out (can tell as the stock tube is already starting to wobble 😣).

 

Either way, get it all back together - test fire on the chrono and nothing - refuses to feed now, looking down the mag well I do reckon the nozzle is not getting pulled all the way back and letting BBs in, but I dunno.. maybe the new tappet spring? pfff.

 

Either way that was me done for weekend, will pick it up again on Saturday - fu gun.

 

  

 

 

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buffer tube bolt into spring guide issue...

 

I'd bet there is a gap between receiver & rear of box

that as you tighten it up it starts to try to pull together

on a plastic receiver it will flex a little as gap closes

but on metal receiver it can't, so it fights and attempts to lift gearbox front up slightly

but the pistol grip is saying hold up and any true alignment starts to throw out inc motor angle

so amps rise & potential lock up may return

 

place a rubber washer at back of box, like a cylinder head washer but not so thick

something like 1mm, maybe 1.5mm tops to take up gap/slack

so box doesn't get pulled backwards as you tighten the buffer tube bolt up

(you could use almost anything - bit of card/plastic, but rubber will squish a bit & fairly durable)

 

the new replacement box is a bit different as you know

and I reckon there is bit more play at back, that throws shit out as you tighten it all up

plop something in the back and it should stay aligned better as you bolt it all up

 

yes it is a bodge, but takes up the play to stop shifting out of alignment especially on metal receivers

like I said some polymer receivers will flex/give slightly before the box is pulled out of whack

 

Rule #1 - change as little as possible - use std box & pistol grips if possible

(alter too much and look forward to more "potential problems")

 

Rule #2 - check as much as possible (binding piston)

 

Rule #3 - go back to basics if all else fails (nowt wrong with this and blame poxy TM comp. BS)

 

Rules #4 or# 5 - send to tech or launch down garden

 

Rule #6 if it ain't broke... no hang on that should be Rule #1 maybe

 

ah fuck the rules, fuck these poxy toy bastid guns

hope you narrow down some these issues & niggles - feel ya pain btw

 

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Changing gearbox shells is never normally conducive to an easy life!

 

Why bother changing the shell!?

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15 hours ago, Sitting Duck said:

 

I'd bet there is a gap between receiver & rear of box

 

Yep can confirm there is a gap, I noted that on Saturday while I was figuring out where I needed to dremel material off.

 

Quote

place a rubber washer at back of box

 

 Interesting, will defo try this - maybe a plumbers washer where it's actually a disc with a hole in the centre would be good (pre drilled hole for the screw)

 

Similar to the above I was also wondering if the the screw being tightened might be pushing the QD spring guide out of it's locked notch into a semi-unlocked position, thereby pushing the spring guide slightly forward and therefore not giving the piston enough room to travel fully to the back of the track.

 

15 hours ago, Sitting Duck said:

ah fuck the rules, fuck these poxy toy bastid guns

 

Yep pretty much mate 😅😡

 

12 hours ago, Davegolf said:

Changing gearbox shells is never normally conducive to an easy life!

 

Why bother changing the shell!?

 

I don't disagree bud, ordinarily I'd preach the same line - but when it comes to this stuff I'm certainly more of a 'do as I say, not as I do' kinda guy 😜

 

Though there is actually a reason in this case; the MK18 is pretty much one of two AEGs I use for both CQB and Woodland, so I do tend to swap out between 330 and 350 a few times a year on this rig. I liked the idea of QD shell for that reason alone.

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That's going the extra mile for 20 FPS! Hats off to you, I certainly couldn't be arsed with that 😂

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4 minutes ago, Davegolf said:

That's going the extra mile for 20 FPS! Hats off to you, I certainly couldn't be arsed with that 😂

 

Yeah, I mean erring on the side of something closer to OCD than logic sounds about right for me 😬

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  • 1 month later...

Finally came back to this GB rebuild and have now finished it to a decent standard - not perfectly however and I'll touch on that.

 

So all of the issues I encountered in the OP have been resolved:

 

On 17/08/2020 at 20:36, Zarrin said:

Reassembled, fired and the piston got stuck right at the back of the shot cycle and the motor whines horribly.

Open it up and notice there is excess plastic on the rear track both sides of the piston - clean that up, assess everything, cycle the system manually, check engagement - everything looks fine.

 

*****

Finished reassembling, and the same again, after one or two shots, piston stuck at the rear of the track and a nasty whine - But it was okay one minute ago! - Aha! maybe the stock screw! - remove the stock tube - everything works again. So I reattach the stock but tighten the screw little by little into the back of the spring guide until I can force the same malfunction - this worked - I backed it out a little upon getting it to break and it seems to work fine - however the screw is already walking it's way out (can tell as the stock tube is already starting to wobble 😣).

 

Diagnosis; as the stock was being screwed back into the spring guide, the spring guide was being ever so slightly dis lodged from its grooves (very odd seeing as the process of screwing that bolt into the spring guide should be pulling the guide towards the back of the GB, thereby giving it no way of dislodging :mellow: ).

Anyway what that meant was as the guide was getting dislodged from the grooves inside the GB, the guide was coming off centre slightly, meaning when the piston was being pulled back, instead of allowing the bearing washers on the end of the guide to pass through the centre of the piston, it was hitting the washers and getting bound on the track.

 

In order to get it installed correctly, i had to levy pressure using a flat headed screw driver on spring guide through the open port of the GB while tightening the stock screw, this ensured that the guide was slotted in correctly and not being pushed out - resolved, no more binding - though I have to remember to apply that pressure on the guide every time I have this gun open.

 

On 17/08/2020 at 20:36, Zarrin said:

Only odd thing at this point is that the shot cycling in semi auto is as follows: first shot finishes at probably 90% pre cock, next shot finishes correctly with piston all the way down the cylinder: these two starting positions repeat 100% of the time in order (mosfet is a very basic King arms no frills, AB or pre-cocking on this thing)

 

This fixed itself by simply using a 7.4v - the 11.1v seems to give this build consistent over-spin. Not an issue as the shimming is good and those ZCI bearing gears are so smooth that the trigger response even on the 7.4 is fast and sharp.

 

On 17/08/2020 at 20:36, Zarrin said:

Either way, get it all back together - test fire on the chrono and nothing - refuses to feed now, looking down the mag well I do reckon the nozzle is not getting pulled all the way back and letting BBs in, but I dunno.. maybe the new tappet spring? pfff.

 

Took the ML Macaron and omega nub out and put in the flat hop setup I was using before - this got it firing again, though with very low FPS about 150 (previously had been doing 330 on the dot for CQB).

I went back inside the GB a few times to diagnose air seal however it proved to be a hop mod which I had made with the intention of improving air seal that was actually wrecking the setup. I had put a tiny O-ring (about 0.5mm thick and about 8mm wide) around the lip of the hop unit which goes inside the GB shell; that tiniest 0.5mm was totally throwing the air seal off - I think that slight gap between the hop and GB shell meant that the nozzle was not sealing correctly with the hop rubber lips when it was all the way forward in the firing position. Either way, took that O-ring off and the power zapped straight back up into the 300s, firing every time the trigger was pulled.

 

 

The thing that I find annoying about this was I wasn't able to get that perfect air seal I was looking for based on spring performance. A couple of months ago I rated all my springs with my MP5 QD build. I ended up using a spring marked 385 on the MP5 which actually has a slightly shorter barrel than the MK18. That spring is giving me 355 on the MK18 - I assume that to mean the air seal on the MP5 is just better 😒

 

Either way generally speaking, I'm happy with where it's at.

 

Anyway, Gonna open up my original CA M15A4 this afternoon and solder a mosfet into it's lonex ultimate GB, may also finally convert it to deans.

 

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3 hours ago, Zarrin said:

I had put a tiny O-ring (about 0.5mm thick and about 8mm wide) around the lip of the hop unit which goes inside the GB shell; that tiniest 0.5mm was totally throwing the air seal off - I think that slight gap between the hop and GB shell meant that the nozzle was not sealing correctly with the hop rubber lips when it was all the way forward in the firing position. Either way, took that O-ring off and the power zapped straight back up into the 300s, firing every time the trigger was pulled.

 

why did you install this ???

I've built up some tape between front of gearbox & hop which is effectively the same thing

or seen posts saying remove hop spring that pushes hop unit back to gearbox etc...

 

these measures are to lightly force the seal at bucking lips to blow slightly

& usage is perhaps if you got a gun just a shade too hot, say 360fps for a 350fps limit

and you know in a month or so the spring will bed in/fade a smidge & hop rubber and stuff gets "run in"

(basically - guns do settle after a short time)

 

so I'll assume you was a tad warm on chrono to add an o-ring like that

some hops do have an o-ring on feed tube (can cause aggro)

and sure I've seen a hop with an o-ring sitting in a groove, to centralise/reduce wobble/misalignment to box

but set in a groove to not push hop away from box, purely for snug alignment reasons

 

poxy guns still

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3 hours ago, Sitting Duck said:

 

why did you install this ???

I've built up some tape between front of gearbox & hop which is effectively the same thing

or seen posts saying remove hop spring that pushes hop unit back to gearbox etc...

 

these measures are to lightly force the seal at bucking lips to blow slightly

& usage is perhaps if you got a gun just a shade too hot, say 360fps for a 350fps limit

and you know in a month or so the spring will bed in/fade a smidge & hop rubber and stuff gets "run in"

(basically - guns do settle after a short time)

 

so I'll assume you was a tad warm on chrono to add an o-ring like that

some hops do have an o-ring on feed tube (can cause aggro)

and sure I've seen a hop with an o-ring sitting in a groove, to centralise/reduce wobble/misalignment to box

but set in a groove to not push hop away from box, purely for snug alignment reasons

 

poxy guns still

 

So before large dial rotary m4 hop units were doing the rounds, I used have a hand full of those red element hop units.

I liked the idea of having that O-ring installed on the lip, always felt like a decent idea for air seal optimisation. However the concept doesn't quite work when you throw one on yourself because most hops don't have that pre-molded groove for the O-ring to sit in, therefore instead of sealing around the GB casing it just gets smushed to the back of the hop unit thereby pushing the hop ever so slightly away from the GB.

 

I do the same though with teflon tape as you mentioned. I just found that random spare o-ring (it came off a ZCI barrel - they come with two of them in the box). Thinking back to the element hop I assumed it might do as good, if not a better job than the teflon.  Needless to say, i was wrong 😅

 

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That o ring is more for alignment of the unit than seal, you dont want air coming that far back (it shouldnt be escaping from the hop/nozzle)

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15 minutes ago, Adolf Hamster said:

That o ring is more for alignment of the unit than seal, you dont want air coming that far back (it shouldnt be escaping from the hop/nozzle)

 Ah du reckon? maybe your right, I always assumed it was an attempt to seal the entire chamber off thereby at the very least improving fps consistency.

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13 minutes ago, Zarrin said:

 Ah du reckon? maybe your right, I always assumed it was an attempt to seal the entire chamber off thereby at the very least improving fps consistency.

 

Nah, your gonna lose it all down the feed tube if it gets that far anyway.

 

Either way, any air leak at the nozzle-hop interface isnt going to help your accuracy.

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3 hours ago, Zarrin said:

 

So before large dial rotary m4 hop units were doing the rounds, I used have a hand full of those red element hop units.

I liked the idea of having that O-ring installed on the lip, always felt like a decent idea for air seal optimisation. However the concept doesn't quite work when you throw one on yourself because most hops don't have that pre-molded groove for the O-ring to sit in, therefore instead of sealing around the GB casing it just gets smushed to the back of the hop unit thereby pushing the hop ever so slightly away from the GB.

 

I do the same though with teflon tape as you mentioned. I just found that random spare o-ring (it came off a ZCI barrel - they come with two of them in the box). Thinking back to the element hop I assumed it might do as good, if not a better job than the teflon.  Needless to say, i was wrong 😅

 

4fa4eca3-b7c3-4f46-a4a3-c00bbce08ca8.thumb.jpg.fc573a5c4c69e1f0ddbd882524fbcec2.jpg

 

yup that is the one - it is designed to slip inside to centralise/improve alignment

not space the hop unit that will drop fps

 

ahhh well you got it sorted now

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8 hours ago, Sitting Duck said:

 

yup that is the one - it is designed to slip inside to centralise/improve alignment

not space the hop unit that will drop fps

 

ahhh well you got it sorted now

 

Yeah, and had quite an old misconception of mine corrected in the process - thanks fellas! :D

 

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Also RE springs and FPS, do the MK18 and your reference MP5 have the same cylinder type?

 

As you go up to higher volume cylinders FPS drops off (all other factors held constant) because the piston gets less run up before it meets resistance - IE compression.

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8 minutes ago, Davegolf said:

Also RE springs and FPS, do the MK18 and your reference MP5 have the same cylinder type?

 

As you go up to higher volume cylinders FPS drops off (all other factors held constant) because the piston gets less run up before it meets resistance - IE compression.

 

The Mk18 is using a 3/4 type cylinder - I'm not certain about the MP5 off the top of my head, though I would imagine Its also go a 3/4. I always try to keep the cylinder type matching the barrel length.

 

From what I understand though the cylinder type should not have all that much impact on FPS, it's more about volume capacity matching the barrel length. 

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36 minutes ago, Davegolf said:

Also RE springs and FPS, do the MK18 and your reference MP5 have the same cylinder type?

 

As you go up to higher volume cylinders FPS drops off (all other factors held constant) because the piston gets less run up before it meets resistance - IE compression.

 

not really, i can see where you're coming from but it doesn't seem to have as big an impact as other factors. aeg's tend to supply a fixed energy rather than a fixed pressure.

 

there is the argument that porting helps fill up the cylinder quicker and the sudden pressure change as the port closes can help get a better piston seal although that's not as much of an issue if you have good static compression.

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Yes other things have more or less impact, but it is still a variable i was offering as to his lost FPS that many overlook.

Lots of little losses add up to bigger losses.

Dynamic / static sealing aside the following still remains;

 

Yes the springs energy is constant.

The mass you are accelerating (piston assy) gains that energy.

With a lower volume cylinder the piston accelerates for longer before hitting resistance, thus gaining more energy.

 

 

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6 minutes ago, Davegolf said:

Yes other things have more or less impact, but it is still a variable i was offering as to his lost FPS that many overlook.

Lots of little losses add up to bigger losses.

Dynamic / static sealing aside the following still remains;

 

Yes the springs energy is constant.

The mass you are accelerating (piston assy) gains that energy.

With a lower volume cylinder the piston accelerates for longer before hitting resistance, thus gaining more energy.

 

 

 

i get where you're coming from, but all i can see it doing is changing the pressure curve.

 

ie a longer but lower pressure curve as opposed to a shorter sharper curve.

 

all things being equal the total energy input to the system remains the same.

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I would imagine @Davegolf is correct about it being another variable that could effect the fps slightly.

But I mean on the same or even greater merit it could be any of the below:

 

- the spring guides which could have a different thickness of bearing washers,

- the air seal of the entire piston/cylinder/nozzle section

- the hop components. Mk18 has a soft rubber flat hopped inside the red metal ZCI hop unit, and the MP5 has a macaron/omega set in a bog standard JG hop unit.

 

The answer is probably in there somewhere 😵 

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thrust bearings for sure will do it, you can even add shims to adjust a little bit. normnally i just run one on the spring guide and not the piston.

 

air seal for sure, hence the discussion about static/dynamic airseal with regards to the porting (although you really want voluming to be correct on the build regardless)

 

normally i'd blame nozzle-hop, as it's much more of a bugger to check than the rest of the system (ie you can be pretty confident by testing the rest of it is right) often its a case of the hop unit sitting too far forward (especially in guns where the hop unit is bolted into place)

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44 minutes ago, Adolf Hamster said:

thrust bearings for sure will do it, you can even add shims to adjust a little bit. normnally i just run one on the spring guide and not the piston.

 

Yeah, I've built and tuned bearing spring guides out of the old one piece ACM steel guides before - very good way of tuning fps actually, you just gotta make sure that the piston will allow any mods you do to pass through freely otherwise you'll get a lockup very similar to what I had with the above case.
 

You actually got me thinking though, you could probably tune fps by injecting hot glue into the piston 🤣 
Maybe lather the canal up with some silicone grease beforehand so you can pull the newly formed rubbery plug out if necessary - but I mean that would probably work tbh 😅

 

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not sure how comfortable i am with the words "hot glue", "piston", "grease" and "rubbery plug" appearing in the same sentence.

 

and it's not just from a purely technical standpoint.....

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21 minutes ago, Adolf Hamster said:

not sure how comfortable i am with the words "hot glue", "piston", "grease" and "rubbery plug" appearing in the same sentence.

 

and it's not just from a purely technical standpoint.....

 

Hehehehe 😝

 

Mate from the purely technical stand point it's bloody genius!  Bagsy, not the one to try it out first! 🙄

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