Jump to content

Advice please on short stroking and premature engagement

Recommended Posts

Hi I am quite new to all this upgrading and have been reading up a lot but cannot find the right answer so hoping for some advice and help.

I have a AK with a 455 barrel


I have put in 13:1 gears and I am using a SP110 spring I have also added an AB mosfet – dean’s connectors – removed fuse and running a 7.4 lipo which gives me a nice 345 fps which is just below my site 350 limit – everything runs ok but not been out and tested it yet only done a quick chrono and getting 33rps.


However, I am concerned that when I go out and do a full day shooting I might get premature engagement due to the low spring and high rps – however I do not want to short stroke due to the barrel length needing the full cylinder.


Do you think I am at risk of premature engagement with the above set up ? – is short stroking an option with that barrel length ? – If I should and can short stroke how many teeth 1 or 2 ? then move up to an sp120 spring?.


I am not needing such high rps was only looking for around 25 but once everything was built it was much higher than I thought so I have some 16:1 gears I can drop in if that would take me out of danger of premature engagement.

Any advice would be greatly appreciated.

Share this post

Link to post
Share on other sites

Your fps is low if that is an M110 spring, and very low if it is a Guarder SP110 which is actually closer to an M120.

At 33 rps and an M120 spring you be fine with regards to PME, but that gun should be managing around 400fps.

Share this post

Link to post
Share on other sites

You used the wrong motor - end of

if you are hitting 33rps then you must have used a high speed motor with high speed 13:1 gears

You are not noticing the overspin due to AB mosfet but you are pushing it

think well pushing it tbh but with AB you are not noticing overspin or near double firing on semi

(reckon at 33rps you will get plenty of overspin if you turned off AB or regular non-AB mosfet used)

Reckon things are running a bit warm - 25 amps or more with AB

use a medium/balanced motor with 13:1's

or std 18:1 with higher speed motors

NOT fast motor & fast gears together - both should get you 24rps or there abouts

a smidge of overspin on a fresh 7.4v but nowt to worry about


Can you short stroke - NO not really coz you need most of that full stroke for a 455mm barrel

So by Shortstroking you will lose volume, if you have corrected AoE then you decreased volume a smidge already

So no I don't feel you can really SS much - a tooth perhaps at a push but not really imho


The SP110 - if that is a Guarder spring then that equates to a regular m120 spring

This would normally place you @ about 393.6 fps & over 400 on a 455mm barrel with a bit of creep

So gun is running with a airleak if you get 345fps - unless that is on heavier bb's than 0.20

(But site will chrono on 0.20's)

So if with a SP110 or m120 you are getting 345 on 0.20 you got poor seal(s) somewhere or something


What motor did you use & what spec/ burst C rating on 7.4v lipo coz 33rps full stroke sp110 or m120 is awesome


I think 33rps is a bit wishful - you sure you hit that much on 7.4v

I have 32-33 on 12:1 SS 4 teeth on a 7.4v 25c with a mental high speed motor

but ultra thick wire and still everything was getting very warm

or 40rps on a dsg with same crazy thick wire and ultra quick motor


What motor, lipo spec, ceramic bearings maybe but not bushings

It would take some work to break past 30rps on 7.4v is all I'm saying

SHS High Speed, Bid Dragon M160, OEM china red nutter motor or perhaps ultra high speed infinity motors

But they will get warm on 13:1 & sp110/m120 spring I would of thought


You do kind of understand all this - so kudos for reading up

but like Hangtight said your spring is a bit low performance so you got a leak

then you will be over so need to SS which you can't

so lower spring but gun will cycle quicker etc....

your motor is too fast imho

change motor or std gears & fit lower spring after fixing leaks

Share this post

Link to post
Share on other sites

Thanks for the replies and help so far ill try to give a bit more info.

As this was my first project I have upgraded everything just because I wanted to see how thinks work rather than try and go for xyz fps or rps.

With regards to the spring this might only be a  M110 this was in my P90 which I was getting around 320fps stock on a 9.6v  – I have got a Guarder SP100 – SP110 and a SP120 I can put in and see what difference it makes – these all arrived late after I put the gun back together and because I have purchased Guarder springs just in the habit of calling the old spring SP110 rather than M110 when really I have no clue what spring it is. Bb’s are 0.20’s

Before I had everything in I did get over spin with standard gears which is why I went for the AB Mosfet and also added what I thought was a high torque / balanced motor, yes you are correct without the AB it double fires on semi

The Chrono was only once for a short burst which did show 33rps – I did not want to fire it again as I thought it was fast and in danger of PE hence the post


·         Lipo is a 7.4V 1300mAh 35C LiPo Square Battery (Deans) Giant Power

·         Motor is ASG Airsoft Ultimate Upgrade High Torque BOOST Motor 40K Softair

·         Rewired with 16awg wire and all deans connectors – fuse removed

·         Lonex 8mm Enhanced Bearing Gearbox for Version 3 – shimmed (the best I can for a first effort)

·         SHS 13:1 Super High Speed Gearset

·         Guarder Cylinder Enhancement Set for Marui AK47 AK47S

·         GATE NANO AAB Mosfet Active Breaking


So no short stroking but should I go back to standard gears or will I be ok to drop in the 16:1 shs set I have ? – i am really looking at getting close to 345 fps and around 20-25 rps I don’t want a mental fast gun that’s going to lock up or over heat 

Also should I swap the motor for something different if so any advice

For reference, I have also added a Lonex hop up with a maple leaf bucking and madbull 6.03 tight bore barrel

Share this post

Link to post
Share on other sites
1 hour ago, madmogga said:


without the AB it double fires on semi



Gun is cycling too fast - end of

you might get away with it (just) and it might appear not to over-run with AB

but if you get a slight misfeed or jam, this will slow the piston on its return and PME


People laugh when you say the gun/machine/car - basically anything mechnical..

The gun is talking to you and yelling you are starting to take the pi$$ now.....


The Spring - well it could be an old faded m110 which may start to work out about right-ish

I mean a gen m100 = 328fps and in a full 455 AK will give you 350fps

or in a shorter gun say AK74 229mm maybe 300fps

TBH think the spring could be about right if you are using old spring like I said

if seals are ok-ish it is probably about a m95/100 rating

The TBB I would forget until the fps drops to say 320fps as you should get a 15 to 20fps boost


What to do.......


Yes you could use 16:1 or std 18:1 with the quick motor

tbh there isn't a lot in it gears wise 16:1 is really 17.25:1 & 18:1 is 18.65:1

(16:1 sets are more like 17:1 - bit of a con ehh???)


You could use a slightly slower motor like a SHS HT which is really a balanced motor at 30k not 40k

(actually think SHS or RA are a bit more like 28k tbh not quite 30k)



This will probably put you about 26 - 27rps and might not double fire

if you drop the battery to a 25c max then you should then be at about 25rps with a bit of overspin but no double firing

(over spin will reduce/vanish with AB)



(this is from EU as UK is out of stock, cut off connector and fit deans 1 LEAD AT A TIME)


Long story short:

fast motor & std gears


balanced motor & faster gears


Hopefully you hit about mid 20's and the amp draw is about 20 amps or just under

A UK spec SSG gun should not really be hitting over 20 amps, maybe a smidge on a high speed motor

But more amps = more heat = drains battery quicker = more wear from heat and not good long term


need a lipo of at least 20c to 25c, larger 35c burst will boost rps a bit and so will the bearings

but tbh it is hard to go above 24/25rps on 350fps without more work on a UK gun

and you can't SS on a full AK

363mm you can SS a couple of teeth, 300mm a few teeth

Hence why the crazy DSG's are for shorter guns up to 250mm:

G36c 250mm, AK74 @ 229mm or stubby M4's etc...


There are a number of ways to dial it back a bit to say 25rps

but your battery will last longer and most likely so will your box

you go back to std slow motor say 15rps and chuck in a 11.1v to get ya say 24rps


As long as your gun stops firing twice on semi with AB disabled you should be ok


Been there myself and learnt the hard way about taking the pi$$ - not worth it

30k motors & high speed gears

or mental motors and std gears - though speed motors do run a little warmer imho

so we usually choose 30k and make the box sing very efficiently at about 20 to mid twenties etc....

shimming just right, piston not binding, seals as close to perfection as possible - makes a big difference

Alas this stuff we have learnt after numerous screw ups

Share this post

Link to post
Share on other sites

Yes, I am learning on the job and have already made a few screw ups and spend a lot of money I did not need too, but to be honest that’s why I chose this gun and to pull it all apart I knew I would have problems but I like to watch – read and learn its  part of the new hobby to for . You have been most helpful thank you for the help

So going to swap the motor for a 30K balanced motor as suggested – use the 16:1 gears I have then pop in the new SP100 and see where I am with that. I think knowing what spring I have and measuring the fps will show if I have issues with compression.

As advised I’ll take the AB out and make sure it does not fire twice on Semi -

Share this post

Link to post
Share on other sites

I'd go for the motor and see what you get

if you use a milder lipo you should be ok


might not need to rip the box open tbh

keep the 16:1 gears and quicker motor for another build say a shorter gun AK74 that you can SS etc....


No point ripping stuff apart if the motor & maybe slightly less juice should do it

work out what battery space you have and could possibly get a couple of 25c lipo's from hobbyking

ensure you order batteries from UK or EU - if you aren't careful you could order from Hong Kong

This will take longer to ship and could in rare circumstances pay import duty

but sod paying £20 for a 35c lipo when you can get nearly 3 x 25c ones from hobbyking instead

Share this post

Link to post
Share on other sites

wow those lipo's from hobby king are real cheap i paid £20 for my 35C and got two of them - feel like i have waste loads of money - i always thought higher C rating was better - what MHA should i go for ?


Also gearbox is already back apart because i smashed the piston - (someone forgot to loctite the head ) so a gear swap is not a problem -

Share this post

Link to post
Share on other sites
17 minutes ago, madmogga said:

wow those lipo's from hobby king are real cheap i paid £20 for my 35C and got two of them - feel like i have waste loads of money - i always thought higher C rating was better - what MHA should i go for ?


We have all paid through the nose for batteries - worst of all is £15+ for a poxy nimah that craps out on ya


Work out what room you have but don't go too nutz - no point buying a battery that just don't fit

(did that a few times)


Get a rough idea of what room you have and take off at least 10mm from each dimension

you need to allow some give for tolerances being out and what about wires/connectors etc....

EDIT - just remembered you may have to take off a bit more due to GATE mosfet etc....


Some guns have loads of space in the stock or battery compartment

others next to sweet FA so the owner has to check themselves

A M4 full fixed stock can get your lunch inside it

A stubby M4 fixed can take about a 3000mah 25c 7.4v or 11.1v

but remember stuff like how the thing slides in/out etc....

the door on the fixed stock is smaller than the battery space so that is what limits the size

(if you are not sure make up a dummy battery shape from card or heck even a Mars bar)

Just don't go nutz on ordering one up way too tight you can't use it


On a modest 20rps to 25rps a 2200mah 25c should last you all day

but I still change it at lunch time to ensure it don't run flat on last game of day

all depends on how much you spray n pray on full auto

how many amps the gun draws

A high speed motor will draw more amps and will for sure drain a battery more quickly

Imagine you are driving on motorway....

You can go steady @ 56 mph for max fuel ecconomy

you can go faster @ say 70 mph coz you want to get home sooner

or you can go nutz at 100 mph, risk getting nicked/busted, run out of fuel if you don't stop to refuel etc...

same with getting a gun running efficiently but not going too nutz


If you have a regular AK stock you should get a 2200mah 25c in there easily

(well I can fit my zippy compacts in fixed AK & stubby M4 stocks)


Should leave ya plenty of room for wires & mosfet - but YOU need to double check yourself

Share this post

Link to post
Share on other sites
33 minutes ago, madmogga said:

Also gearbox is already back apart because i smashed the piston - (someone forgot to loctite the head ) so a gear swap is not a problem -


Bearings on piston head ???

Drop bearings, saves about 4gms in weight to lighten piston

(you have to go nutz to lose that much weight swiss cheesing the piston which takes a bit of time if you are sad)

if you have a bearing spring guide that is all you need - no bearings needed on piston




15mm OD x 5mm thick or 10mm thick if you SS a gun and seek to claw back fps

hole is a tad small M5 size, needs to widen slightly but will save you about 4gms

and less likely to come apart than bearings

YES you should loctite the head, but bearings often come loose very quickly if fitted without loctite


Old cheapo china guns use a great big heavy 9mm alloy lump inside piston @ about 7.5gms

you alter this and fps will vary from extra/lesser spring compression

But you can boost/lessen the fps a little depending on spacers/bearings

What is used or not used can alter the total weight of piston

and a lighter piston is supposed to return a little quicker

(though do not ask me exactly what difference in milli-seconds all this crap makes)

Share this post

Link to post
Share on other sites

thanks again - lots of good information, yes i do have bearings on the head so if i take these off i need to replace them with the rubber washer or do i only need to add the rubber washer to help with tweaking fps ?

Share this post

Link to post
Share on other sites
2 hours ago, madmogga said:

thanks again - lots of good information, yes i do have bearings on the head so if i take these off i need to replace them with the rubber washer or do i only need to add the rubber washer to help with tweaking fps ?


What you can do is something like G&G use on their Combat Machines

(one of the few things they get right)


They use a 5mm thick nylon spacer - well actually 4mm, then topped with a washer or race

The collar then goes through where the old 2 x races + bearing was fitted...


Or a 5mm nylon spacer on its own will be fine (aprox 0.65gms)

saving the weight of 2 x races + bearing (4gms aprox)


Or no nylon spacer or a 10mm nylon spacer with shorter/longer M3 bolt

This will decrease/increase spring tension & fps slightly


This is one of the easiest ways to save a fair bit of weight on piston - about 3gms

To save this weight by swiss cheesing takes time and the work of cleaning up the crappy swarf n stuff inside


Does all this crap matter - depends on your piston

Some pistons like Lonex/Ultimate and certain full metal rack pistons are quite heavy to begin with (19gms)

On a snappy build you should be using a half metal or full metal rack for durability

However - only use a metal rack if you don't take the pi$$ and know your limits

A metal rack piston will have no weak point so if PME occurs teeth will smash - likely bevel will strip or more damage

Plastic teeth will just strip and usually that is all - not such a big threat to rest of drive chain


Do not use SHS 3 metal or 7 metal tooth piston imho

It is a lovely light piston but the durability sucks, I have had two strip quite quickly without PME

And thus I just will never use these again due to the soft blue plastic teeth being too soft

Lovely & light but just not durable


SHS full metal racks are lovely & light & pre-lightened one TT093 is lovely & light (13.5gms)

I don't like using these in boxes with rear inspection windows - feel not much rails are guiding it at speed

But on v2 boxes with no window like G&G std v2's & v3 boxes you got full rails inside box & ideal

weighs about 13.5gms and no swiss crap required

A regular SHS plastic blue piston with full metal rack is about 15.75gms


All this crap is to get the piston's weight down so it should return a little quicker helping to avoid PME

Does it make a great deal of difference - a gram or two not really

A few grams or more should help the piston return a smidge quicker if you are nearing PME

Does it increase anything else - not really

RPS will not change coz that is determined by the sector/drive chain & motor/juice

FPS - hmmmm a wafer of a smidge more but nothing noticeable

Others claim the lighter piston should return quicker so fps will increase but I have not noticed anything great


So - after all this crap is it worth all this lightening bollox ???

To most builds perhaps not - but like the bearings and/or ye olde lump of alloy spring mount (7.5gms)

There are a couple of ways you can save some weight without swissing your piston n stuff

A sniper will want a bit of weight in its piston but a snappy build should not have a hefty piston in there

(others argue a heavier piston impacting on front of box at higher speeds isn't ideal & PME risk etc.....)


You don't have to be a sad bastid weighing pistons & stuff - only real weirdos do that :D

But you can save a bit of weight by dropping the bearings, selecting lighter pistons/heads etc....

Bearings will nearly always come undone if you don't use loctite - so some never use them as it saves weight


I'm not saying you must do this/that - it is your build

I only mentioned the nylon collar as it saves weight and not had one come apart (yet but often I use a drop of loctite too)

If anybody needs piston bearings please lmk - I got more than just a few spares 

Share this post

Link to post
Share on other sites

Thank you again your definitely then man who knows … this has given me lots to think about and do. Ive have placed a few orders for various pistons and heads,to see which ones look/feels and fits the best, I can also weigh them. so far from what’s been delivered the ultimate full metal looks good at just under 14gms, although the madbull looks stronger but is closer to 15gms , at least Ill have a few spars now. Also got a new 30k motor coming (went for the action army infinity) and some nylon spacers so I can remove the bearings. Ill get it all rebuilt at the weekend and see how see runs and post back the results.

Share this post

Link to post
Share on other sites

It's all a balancing act, but there are a few guidelines that are useful to bear in mind when selecting components for a build.

Decide the ROF, trigger response, fps and the BB weight you're aiming for. Will you be mostly shooting full auto or sticking to semi most of the time? How many rounds do you think you might go through in a days play? These are the key factors in deciding the combination of motor, gearing and battery, what type of Mosfet you choose, spring and piston weight...

For the best efficiency you need the heaviest piston that is compatible with the ROF and the spring strength.

Are you limited to the capacity /output of your battery due to space limitations?

As you gain experience then you'll know the questions to ask, and then as you gain knowledge you'll know how to answer them. ;)

Share this post

Link to post
Share on other sites

I'm sure the Ultimate piston is a bit more heavier

I got a blue one with about 10 metal teeth somewhere around (not cheap)

Sure it was heavier than that when I last weighed it and decided nah..... too chunky/heavy for most


End of the day - the weight issue is more of an afterthought tbh....

The main issue(s) is how the damn thing operates - above all else


I usually might have 2 or 3 pistons in mind to use

But often 1 of them might bind in box - it should move freely with box closed up

1 of them might be really sloppy inside - way too much slack as it moves

Hopefully the third one will glide freely but not too sloppy

A piston that binds will rob you of fps, rps and increase the amps as it runs


A SHS might be perfect in a G&G but bind in other different boxes like an APS...

Also have a butchers at this - I don't agree with everything but gives an idea of what crap to check....



Another important fact to check is if certain pistons operate 101% on full stroke

In your AK47 you got a full stroke but some pistons can bottom out on last/release tooth right at back of box

(not a problem if you are going to Short Stroke a tooth or two but your AK & full stroke you need to check)

This Mad Bull piston reminded me of this old problem issue - yes some pistons teeth are set a bit different or more chunky at rear

Thank goodness for Shizbazki.....


What a bloody nightmare eh ???

I mean it is a piston - just slap it in and it will be fine - ergh not quite & not always

So with these issues and ensuring stuff works as it should, any worry about weight is way down the list

So make sure what ever parts you fit do actually work - double/triple check EVERYTHING !!!!


I mean yeah - a bit of weight saved here & there if you can but make sure it operates properly first & foremost

SHS pistons (metal rack not mixed 3 or 7 teeth racks) are a good cheap replacement piston that is quite light

But they don't work in all boxes & no piston should ever be fitted without checking for binding

(learnt that lesson along with many other cock ups of my own)


Piston heads - a good stock plastic one will probably weigh nearly half of an aluminium one perhaps

But this could be 3.5gms vs 7gms to 11gms with bearing, not so much weight issue - what ever works best

seals are "usually" down to o-ring but some heads are crap or crappier than others in operation

So again not so much weight but operation is the key factor really


Once you are content the complete piston is fully functional & effective in operation etc....

Then you could look to see if you can save some weight

lose the bearings perhaps or drop that chunky 7gms 9mm thick alloy spring spacer ffs

Swiss cheesing the piston is an option but know that even if you go nutz you won't save much more than 20%

Often I'd say modest swiss cheesing will save 15% but lets assume we go nutz a second

Bare piston weighs say 16gms, you go nuts drilling/cutting loads off hoping not to screw it up or weaken it too much

Then spend even more time sanding and cleaning up all the whispy bits inside etc....

Whoopeeee - now the piston weighs about 3gms less, you could of saved that by dropping the bearings with nylon spacer


This is the stuff you grasp after a while - yes lightening is a wise consideration but so is choosing the right stuff in first place

We lighten the thing in the hope that it will help piston return a smidge quicker and hopefully help avoid PME

If it still busts after all this care & attention - which it still can coz toys guns = $hit happens at times etc.....

But if/when these bastid boxes goes tits up - well at least I did my best

Open her up, see what broke, learn from it and start again....


I'm still learning with every box, think it took me about 6 boxes to start to grasp stuff a little bit

Shimming was still not great and my first two boxes did not make it back together at all

(my excuse they were cheap plastic JBBG piles of pooh but yeah I really sucked opening up boxes)


As long as the stuff operates smoothly, silky smoothly and in your AK case the piston doesn't bottom out etc...

If you can save a bit of weight - great but not the be all & end all on a snappy 25rps UK gun

Really fast guns then yes every little helps as Tesco say

Also much faster guns have feed issues need delay clips but careful of tappet timings blah blah blah

a 25rps gun is a decent steady build that should work well, not draw too much and last well

(fingers crossed coz above all I still think a little/moderate sprinkling of luck is sometimes required tbh)

Share this post

Link to post
Share on other sites

Sorry Sitting Duck, you are correct when i checked last night the ultimate was not the light piston i picked it was a nuprol




This one is 14grm and already has some holes in the side to save on the weight and looks to be well made, i am also looking to pair this up with the Nuprol Silent piston and Cylinder head 


http://airsoftzone.co.uk/silent-piston-and-cylinder-head-set-aeg-nuprol?filter_name=piston head


new motor arrives today so going to do the rebuild tonight so hopefully these parts will fit and work. if not i have got 5 other pistons and heads to try lol.... because one big lesson i have already learnt is that regardless of what make or part is designed for nothing ever really fits and everything needs a tweak, 

Share this post

Link to post
Share on other sites
20 hours ago, madmogga said:

Sorry Sitting Duck, you are correct when i checked last night the ultimate was not the light piston i picked it was a nuprol




This one is 14grm and already has some holes in the side to save on the weight and looks to be well made.... 


Should be - it is a rebrand like most Nuprol stuff

A rebranded SHS or RA piston...



or SHS piston TT0031


End of the day - you got an idea of things to check & look out for

half decent stuff carefully fitted lasts longer that high end stuff just slapped together in a box imho

You will learn & decide what stuff works/doesn't and will tend to stick with your own preference based upon what works


You ask ten people what stuff to use you will get at least half a dozen suggestions all slightly different

take ya time & you'll be fine is my usual remark - atb etc....


Bit of extra info & crap....


This is one hell of a build from a guru who knows way more than I will ever know....



30rps on 11.1v 4500mah think 25c-40c rating - bit of ooomph in it

in use hitting 15.75 to 16 amps so it is running very well

STD RATIO GEARS - looks like it and on the 30k motor he should just hit 20rps 7.4v

11.1v is another cell so 50% more juice = 30rps blah blah blah to be expected

pulling a m110 spring which was originally hitting 400fps but faded to about 380fps over time

Which is about right as a m110 = 360fps but extra 40fps from longer barrel & perfect seals etc.....

Full stroke but it is right one the cusp coz at very end  when he clears it

It does seem to fire 2 shots nearly on semi - hence gun is starting to talk to you

He is using a heavier spring so piston returns a bit quicker than 345fps UK gun

Has lightened the piston to f*ck - says he did it but looks exactly like TT0093



He has a POM piston head etc... so the complete piston is coming in at about 18gms I'd say

Which is pretty damn good - normally pistons weigh about 20gms or more

Tricky to get a fully complete metal rack piston below 20gms but that piston was 13.5gms anyway

Anyway - light as heck and the amp draw proves it is running sweet

(higher amps = major load/friction/crap shimming etc....but high speed motors draw amps too)


A breakdown of what is inside the box is here:




He implies he swissed the piston - think it was a SHS already done but who cares

I think he should be using a fuller cylinder or type 1 with tiny slits at back for volume

BUT this guy knows his stuff and has probably got the thing shooting just as good as it cycles

So his hop/barrel is probably way better than I will ever achieve too


Still think bearings/bushings should be epoxied into box - seen others say this

But Rogers he refers to is a master shimming god but seen boxes wear with loose fitting bushings


Like I said many will suggest slightly different things but usually sing from similar song sheets of a sort

He is still on the limits at 30rps in my book but he is using a stronger spring @ 380fps etc...

UK wise we are a bit lower so we can't quite push or stuff quite like US can on higher field limits

Hence 25rps is where I feel we need to be careful - overspin or double firing etc.....


Excellent guide is Legacy's guide but any potential readers this side of the pond must remember our lower 350fps

Hence you should not take Legacy's guide as 101% gospel for us limey's

Great advice and tips, clearly showing you don't have to go real nutz on all high end stuff

As long as you take great attention to detail & above all use care & attention rebuilding your box...




Still think these toy guns are like learning to ride a bicycle in a way

Yes watch videos, read up & take advice/help from others etc....

But in the end you kind of just have to have a go and figure it out with practice yourself in the end

Though it does help if you have half a clue of what you should be doing in the first place

(I did not and fell on my ar$e many times if truth be told - not quite as easy as it first looks)

You will be fine, 25rps- ish running sweet & not too nutz etc.....

Share this post

Link to post
Share on other sites

well just finished putting everything back together, glad i had a mix of pistons and heads as some did not slide easy or give good compression, need to strip it back apart again as it is running way to hot for my uk site  - results using a 30k motor on 16:1 gears and a 7.4 lipo on a sp100 spring with my 455 barrel gave me 375 fps and 16 rps air seal seams good as the fps only went from 374 - 376 over about 40 shots. might pop in the 40k motor again and see if the FPS is closer to 20 / 22 


i need to get the fps down to around 345 - so should i get a sp90 spring or remove the 5mm nylon spacer from the piston head ? i am not sure but i think removing the spacer might only drop me about 15 fps just want to avoid stripping it apart another 20 times !



Share this post

Link to post
Share on other sites
6 hours ago, madmogga said:

well just finished putting everything back together, glad i had a mix of pistons and heads as some did not slide easy or give good compression, need to strip it back apart again as it is running way to hot for my uk site  - results using a 30k motor on 16:1 gears and a 7.4 lipo on a sp100 spring with my 455 barrel gave me 375 fps and 16 rps air seal seams good as the fps only went from 374 - 376 over about 40 shots. might pop in the 40k motor again and see if the FPS is closer to 20 / 22 


i need to get the fps down to around 345 - so should i get a sp90 spring or remove the 5mm nylon spacer from the piston head ? i am not sure but i think removing the spacer might only drop me about 15 fps just want to avoid stripping it apart another 20 times !




Woah - that is quite a difference to what you had.....


FPS 345 to 375 is a 30fps jump, you must have improved the air seal a bit with different piston head/o-ring

Plus is it the same spring and is it an irregular spring - tighter coils at one end than even all the way ???

If it is an irregular spring - did you fit it in reverse ???

The tighter coils go at the back of box on the spring guide

if you fit tighter coils up at front of piston - this can increase fps by about 10-12 fps I have found


I'm assuming you already had a bearing spring guide at the back previously this adds about 15fps or so over std guide

(normally fitting a bearing spring guide & losing a smidge when correcting AoE kind of cancels each other out)

The same spring - which quite likely could be a m110 faded to m100 will give out about 350-375 in a AK

But same spring will give 300fps in a mp5k with much shorter barrel - so a m100 rarely hits 328fps in every gun

Factor in final spring compression which with bearings/spacer plus bearing guide will be 2 x 4.5mm extra compression

(compared to std spring guide and zero piston spacer/bearing/spring mount etc...)

Each tooth is about 3mm so you have extra 3 teeth compression but you can't really short stroke (maybe one tooth at a push)

YES you are correct the spacer will drop about 15 fps - same as a bearing spring guide

Also is the spring fitted the correct way - tight coils at the back of box if an irregular spring ???

Yes you have gained 30fps maybe this was say 20 fps increase improved seals

10fps boost from either back to front spring or crazy light piston - though think 5fps myself on lightened piston


The ROF though - WTF has happened to that ???


First of all you are on bearings which will give you an extra 1 or two rps perhaps over bushings

Secondly you are using 16:1 over 18:1 gears - which though not true 16:1 but 17.25:1 ratio

You should still get about an extra 8% over std gears rps wise

On a great day, perfectly shimmed, lovely spinning gears on bushings or mix of bushing/gears

with thicker 16awg - and the final quality of wire/its CSA/how many breaks in wiring loom deans blah blah blah

But on a great - all boxes ticked 30k motor 18:1 setup on a 350mm barrel 350fps you should be @ 19-20rps


So my main hunch is that 30k is not quite 30k - more like 25k tops imho


If your fps wasn't so high I'd say go back to the 40k motor but you gotta open the box to drop fps

I mean yeah - whack in the 40k motor and you should get around 24rps to 26rps

But still gotta look into dropping fps - but give a go by all means....


If you can use the 40k motor as is for now to get say 24rps then great

Did say just get a SHS or RA motor would be an option at the beginning....

BUT the bearing came adrift, so you could have still got a fps boost when you fixed up the box anyway

So no point blaming this or that or should/shouldn't have done that - results do vary a smidge on each build/opening


Try the 40k motor with the 16:1 gears

13:1 gears + 30k max motor

std or 16:1 gears + faster motor - so go with this perhaps


The fps - you have numerous options

SP90 = M100 or a m95 if you got awesome seals atm

cut a couple of coils - don't like this coz it is still a bit of guesswork plus need to crimp flat & finish end of spring off

remove piston spacer and perhaps the bearing spring guide at back going to std spring guide

would be wise to still have a bearing somewhere for potential spring twist but need a gun with UK limits to use

maybe remove just one - say spacer and rotate the bearing collar or something to use existing M3 piston head bolt

(or shorter M3 bolt & washers etc....)

ensure the spring itself is fitted tighter coils at rear - this is an easy mistake and can make a 10fps difference

So if the spring is ar$e about face you could fit it correctly and tbh use std spring guide.....

This should get you say 25fps less = just over 350fps

You got a metal rack piston, leave gun half cocked or pre-cocked overnight is just over 350fps

(assuming your chrono is correct as sites chrono is what they decide upon btw so careful if you got 349.9999fps)


OK spring twist OMG - relax, in 6 months time your gun will drop a bit fps wise no doubt...

when it hits 320-330fps from wear on seals or if spring fades a bit - pop the bearing spring guide = 15fps extra


Problem buying another spring is you then start with a blank sheet of paper....

You do not know for sure what exactly your present spring is to begin with - likely a m102.5 ish

Then the new spring could be this or that....

An APS UAR gun has one of the quickest spring change I know of - in about 90 seconds you chrono two springs

It is a great way of comparing two springs to greatly reduce the guesswork

Though not to be taken 101% exactly as it has a 400mm barrel aprox it is more the comparison it offers

So the difference should kinda correlate if not exact fps in a shorter gun then aprox percentage wise

Alas all this maths bollox never quite works out 101% as intended but hopefully gives a modest guide that just pot luck


Yes you could try a m95-ish spring and it could work out about right....

I'd consider working with existing spring one last time if you check to see if losing a spacer/guide & spring correctly fitted

If still a nightmare then look at hacking or buying another spring

It isn't the cost - a tenner is nothing but more you could still be guessing the final results and another strip down


I'd try the 40k motor to see the rps on the 16:1 set - should hit 25rps, if so the new motor is 25k max imho

(use new slow motor with 13:1 gears blah blah blah)

then open box - check spring is fitted correctly, look to lose bearing spring guide and spring correct way

(tighter coils at rear)

If spring is fitted correctly then you would have to lose bearing spring guide AND spacer tbh....


However - there is also another possible reason for fps change if you changed piston head and cylinder head....

Is the AoE still corrected properly - on some silent heads that are concave/convex shaped you can't really correct AoE

Well not like usual heads - now if the AoE is no longer corrected like before you have a slightly longer stroke perhaps

The longer stroke increases fps - we lose about 10 to 15fps when correcting AoE but bearing spring guide compensates etc...

If the AoE is now slightly under corrected from new heads etc... then the stroke will add a bit onto fps too

How do you correct AoE on those heads - with a spacer behind the piston head - say a 15mm x 2mm/3mm tap washer

Won't this bottom out - no coz actual piston retraction does not change the piston head stroke is shortened a whisper


This is now clutching at straws but trying to cover every possible aspect of how the fps has altered

Most likely down to great seals etc.... but just slight alterations to present setup can make a difference

I'm just trying to list everything to consider before you end up launching the thing down the garden

(been there quite often - actually very often if I'm honest)


The fps if still the same spring could be fitted back to front and maybe a couple of other things like heads chaged

The motor is way out imho - think it is 25k - try the 40k first and see what you got...

Have a think exactly what you have done and give yourself a bit of time to reflect what exactly changed fps wise


Sorry for massive waffle and no doubt continual repeating myself but trying to cover everything

hopefully one last opening and attempt will achieve the rps & fps you need to get to




If you are on the 16:1 set then you could or should be able to use old std sector gear...

It "should" be compatible with the 16:1 spur gear and if it runs smoothly

and the last tooth on sector is not a fat final release tooth...

You "could" experiment with ShortStroking the old 18:1 sector gear one tooth if you are stuck

Note some sectors have a fatter final release tooth on sector and so you you need to grind off on piston

But if all the teeth on sector are exactly the same then you can leave the piston's teeth untouched

pic says a thousand words - see the sector on the right - the fat final tooth is what I mean....

Image result for airsoft sector gear

IGNORE THE SECTOR ON LEFT - this was ShortStroked incorrectly

They took off five teeth (jeeez) mostly on the WRONG SIDE which screws up tappet timing

(16 - 5 = 11 ffs, a dsg has 8 or special dsg has 9 teeth but fires twice so could have gone dsg)

(you have to shorten tappet fin and increase tappet spring compression etc.. to compensate for this)

You take off a tooth or two on the other side first like this....

Image result for airsoft sector gear


I messed up following an incorrect guide when I first started

not saying going nutz short stroking coz you need full stroke or nigh on for AK47

but you "could" perhaps experiment on old std 18:1 sector is what I'm saying with 16:1 set

(you get the idea of what I mean - can't do this on 13:1 coz sector is different)

Just be careful if std sector is chunky last tooth as you would have to remove the piston's last tooth

(not worth it but just a thought if the sector's teeth were all the same blah blah blah...)

Share this post

Link to post
Share on other sites

wow that a big post but thanks, sorry i should have explained that the sp100 in the above post is a new spring i thought i would put this in as i had no idea what the old spring was, knowing i had an sp100 means i now know everything in my set up and can make sure it is performing as it should..


so to get the fps back down i did put in the old unknown spring again this has now dropped the fps from 375  back to 348 - 350 this for me is still a bit too high as my site limit is 350 so as you said depending on their crono i might get rejected and would like this to be a more consent 345 if possible .


as for the rps i am starting to think my initial reading of 33 rps was wrong or a miss read, however my rps is still causing me problems / concern now - on the 30k  motor i was getting on average 15.3 rps - so i swapped back in the 40k motor to see what difference it made and i am now getting 17.6 for me i would expect this to have been closer to 22rps so i think i do have a problem somewhere that i  have not spotted.


i have adjusted the AOE for the new piston and head, re shimmed for the new 16:1 gears (Checked to make sure these ran free when gearbox was screwed down etc)  - checked motor height ( i do not have a lot of adjustment and anything i did did not seam to make any difference) - checked piston moves freely - checked i am getting 100% stroke  - check compression - re greased gearbox (all to the best for a newbie) - spring with tighter coils installed at the back - bearings on the spring guide and nylon washer on piston head as recommended.


apart from still needing to drop about 5 fps to make sure i am well under my site limit i am very happy with how the gun shot - good trigger response etc. i am not fussed about getting the rps up but just surprised and want to make sure ive not done anything wrong or missed something stupid that is going to break something at my first skirmish.

Share this post

Link to post
Share on other sites

Ahhh - see that is what I said about new spring = new piece of paper....

OK you are at 348-350, adjust the hop a smidge and it should drop a little

Also if you set the hop a smidge further for say 0.25's then when chrono'd on 0.20's you will be ok

This is not cheating the chrono as you are using 0.20's to chrono on a hop set for 0.25's

So unless the bb's are going mega sky high straight away it will pass the chrono

very few places will demand the hop set to zero or we would all be 15fps over the limit



Right the shimming is out of whack then when you changed to 16:1 gears perhaps ???

No way should a 16:1 gears & 40k motor setup be shooting that slowly - no bloody way


On a v3 it is a tad easier to check & set the bevel height to pinion

place bevel in top half of box fit the motor cage up and check it meshes back on just right.....

see this long post about shimming stuff....


Odd thing is the bevels for the SHS 13 & 16 sets "should" be the same so it "should" be still correct unless you swapped stuff

Even so you should check the height and such......


Then gauge what shims required on top of bevel - forget under the bevel

the pinion will keep the bevel in place so it won't drop down

the bevel is the only gear you don't have to shim most of the slack out of it

just get the height of motor and height of bevel - determined by shims on top - get these correct

if there is a load of slack underneath afterwards no biggy - take up the worst of this but a bit more slack is fine

(the pinion holds the sod in place like I said)


Space out the other two - usually sector is often shimmed half & half say 0.20mm  on each side

(but you got bearings so this rough amount might be 0.35mm on each side

Spur is usually shimmed about 0.20mm under neath and what is needed on top




Every box is different, SHS gears are quite chunky and bushings needs much less shimming than bearings

So do not take any of this as gospel - you need to check all the stuff yourself

But the motor height must be set, then bevel shimmed to the pinion


Something is well skewed on the shimming most likely to be losing rof like that

Honestly that f*cking motor & bevel crap is so damn critical to how the box runs....


But the bevels for both sets of SHS's should be the same I would of thought ???

should be the same bloody bevel gear I think so I don't how it has all gone tits up

The gun must have been going nutz at 25+ rps originally and double firing with AB turned off


I mean std guns will shoot about 12rps

deans + rewire + mosfet would boost by about 15% so that equates to say 13.8 rps or 14 rps

16:1 gears will improve about 8% to get to 15 rps on $hit stock motor say 20k

then use a 30k motor so we go a third faster again = 20 rps

or 40k motor gets up near 25rps on 16:1 or 30+ on 13:1 gears


So that is where the speed figures come from along with previous rough builds done right

Now all guns will vary but your spring is a faded one on a long barrel hitting 348fps

this would most likely be a 320fps medium length build gun or sub 300 mp5k using same spring

So that spring isn't insanely too strong, so you should be hitting those figures in my mind


So that rof is well down imho - check the shimming thoroughly coz that is where you are being robbed

Share this post

Link to post
Share on other sites

Daft question.....


You mag is feeding 101% ???

same mag, same nozzle & stuff


not being funny but gears are not shimmed right or bb's are misfeeding a bit to see the rps drop like that

eg: 1-1-1-0-1-1-1-0-1-1-1-0-1-1-1-0-1-1-1 etc... to see the rof drop out of the ordinary ???


something ain't quite adding up to me

did the 13:1 set have a delay clip or something to assist feeding

has nozzle changed - large nozzle that doesn't clear to chamber bb

worn out tappet fin not retracting the nozzle back far enough to feed properly again

using a different mag with a weak spring not wound enough high cap

if mid cap - don't be fooled they always feed better than high caps

(had a couple of mid cap M4's that bb's were snagging in the bends on the mag/spring and thus stopped feeding 101% at times)

Jeez that was a nightmare - stripped the gun twice more before I realised it was the f*cking mid cap I was using

jeez the air was dark navy blue that day

Share this post

Link to post
Share on other sites

I think it was feeding ok but I'll check that first as I did change the nozzle but from what you have said I think it's my shimming it's only my second attempt and although the gears mesh ok and spin free with the gear box closed I must admit I did not even shim for or check the motor height dammit I knew I would miss something 


Share this post

Link to post
Share on other sites
3 hours ago, madmogga said:


I think it was feeding ok but I'll check that first as I did change the nozzle.....





Or if you do, then do two things:


1) double/triple check old part with existing part - there is at least two AK nozzles out there - long/short

PLUS certain manufactures make stuff slightly different so there is scope for stuff not to operate 101%


2) include what other bits you changed or altered as well - coz I'm having to include extra bits n bobs

Won't be the first time a thread continues to grow & grow - and this is growing even by my standards...

only to find out other facts emerge that complicate or change the initial issue/problem


The tappet plate moves back n forth to feed & chamber a bb

a nozzle is fitted that works with than gun/box/hop unit etc....

you only have about 7 to 7.5mm movement for tappet to retract and allow a 6mm bb to slip into hop

Also this needs a bit more than so the nozzle is pressing against the bucking lips to seal correctly

if it barely touches the bucking after chambering a bb the seal will blow

If you fit a nozzle too long then when it retracts it might not have just enough room to feed & chamber a bb into the hop unit

or not reliably 101% of the time - so when ever you think of replacing stuff you MUST CHECK & COMPARE stuff first....


Changing spring & nozzle "may" only add additional factors into the mix that was kind of ok

spring was 345fps originally, the box opened due to bearings coming adrift from piston head

(I suggested nylon spacer to save weight easily and less chance of working loose like bearings etc...)

Old nozzle could have worked lovely if you did fire at 33rps

New nozzle or new setup "could" have a feed issue or sector/delay on different sector gear on other gears

Hence did say could have just used the 30k motor instead of 40k motor - keeping it simple-ish

You was likely hitting 33rps on a 40k motor...

a 30k motor is 75% of a 40k motor (40 x 0.75 = 30 or 30 x 1.333333 = 40)

So take 33rps and multiply by 3/4 or 0.75 = 24.75rps for a 30k & 13:1 setup

Which is what was said at the start - the 40k motor was too quick with those gears

Simple change of motor, keep gears etc.... fix bearings/spacer (fps hardly changed on old spring)

Should have fed ok - well if it fed on a test @ 33rps then it should feed ok at 24.75rps so to speak

(of course all this basic maths bollox is only ever a very rough guide but there abouts)

minor work & little hassle messing about with any more stuff

piston was optional as I didn't know what was in there already or its durability

the bearing to spacer would save a fair bit of weight without going nutz swiss cheesing (more time/hassle) 


It is unlikely - but not completely impossible the 33rps was a misread on chrono but think it was correct

Normally if you are shooting at 33rps you are shooting at 33rps

if anything if you have a bb misfeed in stream of say 15 round burst then it will not say 33rps but say 24rps

Which if you are expecting 25rps then a misfeed issue drops it to say 18rps etc.....


I mean you could record the gun firing on your phone the play back the sound clip in audacity counting the spikes

Or you can use an app like - Paintball BPS meter

Though note it is fussy according to sound of your gun/muzzle/dry firing you bare box+motor

it might say 83rps on one test, then the next test it might say 23.75 the next time

If you have a rough idea of what to expect it does work well - when it actually works properly

It sometimes picks up the shrilly sound like a sewing machine that most AEG's sound like = odd results

Just saying if you record the sound or run the app it might indicate a different rof to possible misfeeding at chrono


Thing is with these bastid boxes you really don't want to be changing loads & loads unless it really needs it

AND when you do you must really really check everything as you go...

Forgot all these guru's making crap look easy - it is not that easy until you have f*cked up loads & learnt from mistakes

That is wisdom - the term wise after the event etc......

Don't complicate stuff too much by chucking more stuff into the mix all in one go

Even Legacy says you don't need to go nutz for modest builds with new aftermarket cylinder heads if ptfe tape works

Std piston head can be fine but just needs a better o-ring (though some are crap)

Nozzle yes often I'll change for an o-ring but check & compare the original against new nozzle for compatibility etc....

Tappet plate I might change only if say it doesn't hold the new SHS nozzle - like Cyma M2 boxes/tappets on SHS red M4

measure the thickness at front of tappet plate coz they do vary and might need sanding - not for feeding but sealing

(don't worry about this or think I'll change this as well - just saying a lot of crap is different and may not work as is)

So why bother adding more money & work checking/adjusting stuff you may not need to as well as add more grief ???


I'd be tempted to try and record or test the sound of box in app

see if gun misfeeds to arrive at low rof at chrono

if there might be a misfeed issue then you can open box, check the motor height/shimming and perhaps replace old nozzle

Or at very least get them both side by side or on table and stare very carefully at then side by side etc....

(digital calipers are a great measuring tool for checking stuff like battery dimensions, nozzle lengths and of course shim thickness)


I never said it was easy - but please stop keep changing stuff too much coz you are only making more work or confusing matters

Share this post

Link to post
Share on other sites

Ok sorry to keep confusing things I am learning about guns and forum posting at the same time. With the nozzle I did use a digital micrometre to check the length and checked seal and that the tappet return allowed bbs to enter hop up ok so I am 99% sure that feeding is not the issue and that shimming is. I changed the nozzle because I was not 100% happy with the seal on the old one so swapped it to one with an o ring. Might not get a chance to strip the box down again for a few days but ill post back on my long learning experience later in the week. Like I said at the start I am only doing this to learn so all these issues are great in one respect as I am getting to know all the problems and how to solve and check all at once.

Share this post

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...