AK74 build (not custom at all!)

[banana90]

This is what I have so far. Gears are not shimmed yet and sorbopad + rubberpad are not installed, however I had superglued the bushings and gearbox had cylinder walls already radioused.

 

Here is a list of parts from the picture.

 

Guarder SP110
Guarder steel spring guide
G&P Polyacetal High Pressure piston
G&P bearing (for piston head)
G&P spacer (nylon)
SHS aluminum piston head
SHS stainless steel cylinder head (double o-ring)
SHS stainless steel cylinder (full type)
SHS aluminum nozzle (o-ring)
SHS steel bushings
SHS 13:1 gears
SHS shims
SHS steel ARL
JG Blue motor
Sorbopad 70D + Rubberpad

 

What you don't see in the picture:

Silverplated wire

Gate NanoASR mosfet

Deans connector

Maple Leaf 60D bucking

Maple Leaf flatnub

Madbull 603 455mm Steel TBB

 

More will follow. Target is to build a gearbox with great reliability, nice trigger response, max. 400FPS and around 20rps. Nothing amazing, but like always, I pay attention to details and assemble the gun slowly, with precision, leaving no marks that it was ever touched.

[ninja master of coffee]

It is looking good so far, are you planning a shooting vid afterwards for things like, range, accuracey and trigger response? I'm impressed by the quality of SHS parts considering how affordable they are.

[banana90]

I will make a video if there is an interest.

 

SHS parts are really nice IMO. SHS cylinder, cylinder head and nozzle give perfect 100% compression - found out in one of my previous builds. When I had put my finger over the nozzle (in extended position while being attached to tappet plate and whole assembly was in GB) I couldn't move piston forwards, not even when I tried with my bodyweight. I was also testing piston heads from SHS (alu) and G&P (explosive) and SHS gives far better consistency (SHS was +/- 5FPS while G&P was +/- 15FPS). Therefore, SHS is my number one choice when it comes to compression parts. Guarder is also very nice, but at almost double the price, it doesn't have better performance.

 

For gears, RiotSC is ofcourse the best, but my setup is not extreme and such strong gears are not needed. I heard SHS also makes nice gears (eventhough not comparable to RiotSC in strenght) so I went with those.

 

At first I wanted to build a DMR, but since I would be limited to semi only, I decided to go with full auto assault rifle with 400FPS limit. However I may still get a scope if the gun turns out to be accurate at distance of at least 50 meters.

[ninja master of coffee]

I recently swapped the standard piston and piston head of my CYMA AK74m with the SHS 15 tooth full steel piston and one of the bearing piston heads. Added a guarder SP 100 spring also... Now I need to swap to a softer spring because it's doing 344fps with a .25g BB (so I guess it would do about 370fps with a .2g) I'm currently planning on using an SP 90 spring, any suggestions? Site limit is 350fps with a .2g or 313fps with a .25g.

[banana90]

I would first try to install sorbopad. It will reduce your cylinder volume and therefore reduce your FPS output. You will also benefit from better AOE.

 

If you have sorbopad already installed, than you could shortstroke your sector gear (would be wise if you have big ROF and weak spring) or simply replace your spring for SP90.

[ninja master of coffee]

Cheers! I think I will go with swapping the spring for an SP 90 as it seems like the simplest solution. If the FPS drops more than I like I can always replace the cylinder and cylinder head as they're still both stock. That said the air seal around the cylinder head seemed fine when I checked it and I suspect now that the O-ring round the piston head has properly absorbed silicone lubricant it's running pretty efficiently, what FPS would you expect out of an SP 100 if compression is good?

[Tarr]

I have done something similiar with an M4 very recently, I went with Lonex compression parts instead.. was very impressed considering the price.

[bankz5152]

Nice build! 

[banana90]

Yesterday I did the shimming and found out that SHS sector gear is not meshing good with G&P piston. Teeth are not making enough contact, only the tips of the teeth are touching (about 2mm).

 

When comparing SHS sector gear to standard XYT sector gear, the second one not only has a bit wider and longer teeth, but SHS teeth are also a bit curved at the edged. It appears they have a unique design that only SHS piston should be fully compatible to. I will visit a store today with my gearbox to check the fitting with SHS piston. If it meshes nicely I will buy it, if not, I will return the gears and go with another set.

 

I am also planing to get SHS pinion, I heard it meshes with bevel gear better and makes less noise than standard pinion on JG blue motor.

[banana90]

UPDATE: SHS 15 steel teeth piston meshes like a charm with my gears. Pinion also meshes perfet with bevel. This should work nice together.

[banana90]

Here are some pics. It shoots 350FPS with .25g at 15rps on 7.4v 1300mah 25C battery so nothing impressive there. Just a reliable setup which must meet 400FPS filed limit and not being a spraying gun. Trigger response with this motor and gears is amazing! It is accurate up to 60 meters, but I will install R-hop anyway.

 

[banana90]

It is finally done.

 

Here are the specs:

 

Internals:
Guarder SP110
Guarder steel spring guide
SHS 15 teeth piston with full metal rack
SHS aluminum piston head (w. G&P bearing and G&P nylon spacer)
SHS stainless steel cylinder head (double o-ring)
SHS stainless steel cylinder (full type)
SHS aluminum nozzle (o-ring)
SHS steel bushings
SHS 13:1 gears
SHS shims
SHS steel ARL
JG Blue motor with SHS steel pinion gear
Sorbopad 70D + Rubberpad
Silverplated wire
Gate NanoASR mosfet
Deans connector
E&L stock bucking
Maple Leaf flatnub
R-hop
Prometheus EG 603 455mm Steel TBB
SHS steel hopup adjustment lever

 

Externals:
US Palm pistol grip
US Palm 7.62x39 mags (I know, I know, lets pretend it is AK103)

 

It shoots 330FPS with 0.3g which is around 400 with 0.2g. ROF on 7.4v 1300mah 25-40C lipo battery is 16rps. It cycles extremely smooth and silent and I can easily hit a man sized target at 240ft. Overall I am quite happy with its performance.

[ninja master of coffee]

AK 103s are just fine! I have a couple of questions as I'm also in the process of upgrading a CYMA CM040 series (AK74m, now an AK103 with correct top cover and magazines). I found it had compression problems with the stock parts and dropped as low as 230fps with .20g BBs so I swapped to a Guarder SP 100 spring, SHS bearing piston head and SHS full steel 15 tooth piston, that worked a bit too well, putting the FPS up to about 380 with .20g BBs. I have now swapped down to an SP 90 spring (the site I most frequently go to has an indoor limit of 330fps and outdoor of 350fps for select fire weapons, I would prefer 330fps for flexibility). Do you have any idea what FPS I can expect from it? Sadly I don't own a chrono so it's always a case of waiting till a game to actually get anything tested. I have also ordered an SHS double O ring cylinder head.

 

Cheers.

[banana90]

380FPS on SP100 sounds too high. What chrono did you use? Are you sure it makes correct readings? I would check with another chrono first.

 

Also I wouldn't recommend going below SP100/M110 spring. I would lower my FPS with shortstroking.

SP springs are rated with 0.25g and 100 stands for m/s. Do some calculations to get the expected FPS output. You should be around 360FPS with that spring.

[ninja master of coffee]

Yeah I was quite surprised. The chrono is the site chrono from where I normally play, as far as I'm aware it is accurate but I find chronos are quite variable. It was actually tested with .25s and was doing around 340fps -350fps (I think) with those so the 380fps was an extrapolation from there. I guess it may have been a combination of somewhat higher than expected power output and power creep with the .25s by comparison with .20s but then I skirmish with .25s anyway. What fps would you expect from an SP 90 spring with good compression? Thanks for the info!