Dragunov issue (air leak?)

[Gretik]

Hello,

I've done what research I can, but I'm come up short! I have fitted the airsoftpro.cz parts, pretty much the full set, to an A&K SVD. It functions, but the FPS has stayed very low, given the power of springs I have fitted.

I've used a variety of springs, including a 190 - it only generated 400-520 FPS (with a very wide separation between the various results). From what I can tell, the compression of the piston is solid but I believe air is leaking in or around the hop unit.

Any advice would be much appreciated!

Cheers for reading,

Daniel

 

[RR01]

First thing is did you seat the new cylinder head fully into the slide and did you fully tighten it into the cylinder itself (you did say you fitted a full set of parts)? The front of the head should then be flush with the front end of the slide where it stops being round. It takes a bit of a tap with a hammer to fully seat it, as it's such a tight fit. If it is fully inserted check, without a mag inserted, that the nozzle then fully enters the hop unit (you should see it pass through the BB feed hole from the mag) and that there is a slight grip from the hop rubber once it is fully seated and as you try to pull it back (without a spring fitted obviously). If all this is correct then either your hop rubber is faulty or badly installed, or the piston O ring isn't sealing as well as it should (although your FPS figures don't suggest this).

Other than that I can't think of anything else, as I've done exactly the same upgrades to mine and mine fires consistently in the high 400s on a M150 spring.

 

[Gretik]

Tomorrow, when I'm a bit less irritable with the whole thing I'll take it apart and try your advice - I tapped it in the first time, but hand fit it the second.

Also found that the piston head has some odd pitting on it, but only along the front face of it (along the metallic rim, rather than the rubbery bit) - the sides are still smooth.

Cheers,

DanG

 

[RR01]

The rim of the piston head should be smooth, so that there is nothing to cause any scoring on the inside of the cylinder. Mine is smooth, although I think I may have the Mk1 piston, not the upgraded one. I'll have to check next time I take it apart.

 

[Gretik]

The rim in contact with the cylinder is smooth, there are small dents or pits in the face of the piston head - looks like possibly something was caught in it and dented it when it was fired at some point. Ptretty dumb on my part if that's so, but it shouldn't be affecting the air seal (tested the compression of the cylinder alone and it was fine).

 

[RR01]

In that case the only thing I can think of is that the inner barrel isn't a correct fit in the new hop unit, or the hop unit isn't assembled correctly. I know I put mine together slightly wrong the first time, so it could be that.

 

[Gretik]

It's a generic madbull 6.03 tightbore, could always try it with the stock barrel again if needs be.

As for the hop up, it currently has the stock bucking in it as I wasn't confident that the softer blue bucking from the cz kit wasn't getting squished or displaced when I inserted the barrel.

(It certainly didn't feel loose.)

That said, I may attack it with PTFE tape.

 

[RR01]

There should not be a problem with a Madbull barrel, but they can be a tight fit in some hop units. I've put an Element 6.01 x 600mm in mine and that fitted fine, although I have to use very good quality ammo in it.

If you are taking it all out again, make sure that all the grub screws are making proper contact with the slots in the inner barrel and within the outer barrel itself, as if there is any slack here it could give rise to some leakage.

As regards the hop rubber, if you are aiming for around 500fps then you definitely need a harder hop rubber than the standard AEG ones. Something like a 70% or 80% hardness would be best. I've got a Falcon twin point hard rubber in mine and that seems quite good, although I do need to "shoot it in" with another 1K rounds or so before it settles down fully.

 

[Gretik]

Well I swapped the piston for the stock one, and realized quite how bad the seal on the 'new' one was. The O ring on it must just be totally the wrong size.

FPS is now 500 with a 150ms spring, I'm going to take off the new brass spring guide and replace it with the stock one as it's a better fit for a 150 spring - then open up the hop and add a bit of PTFE to keep it sealed nice and tight.

Will post how it goes The fun and tribulations of setting up a sniper...

 

[Gretik]

Tried padding the o-ring groove of the 'new' piston with PTFE, as well as the screw holding the bearing but it still leaked - can't see any scratches on the rim and my second attempt at padding out the o-ring left it unable to even fit into the cylinder, so I'll probably look at getting a new one.

Meanwhile, applied PTFE to the hop rubber where it meets the barrel and that has improved the air seal of the barrel enormously. I don't think the cylinder head and the hop up rubber are forming a perfect seal - but it's currently pegging 500 very reliably. I now need to trim the spring slightly to bring it down to more like 480/490 - it is occasionally wandering over the 500 mark.

(I now both love and detest PTFE tape, fiddly damn stuff. I thought grub screws were bad...)

 

[RR01]

Sounds like you are getting there. Personally I would not clip the M150 spring. Just put it in the gun and leave it cocked for 12 hours. That will weaken the spring slightly by being compressed for a long period. Try running it through the chrono then and you should see a slight drop in FPS. If it drops too much, take it out of the gun and let it settle back completely uncompressed for 24 hours, then try it again. I left an AEG spring compressed in error for 8 weeks and saw a 40% drop in FPS, but it pulled itself back to about 85% of normal after being released.

I think the new AirsoftPro piston is supposed to be a slightly looser fit in the cylinder, but the O ring should be oversized to take up the difference. However, if it is a little loose, try removing the O ring and stretching it over the cylinder for a minute or so. Seen that done in a tech video somewhere online recently. Supposed to stretch it enough to take up the slack and reseal it.

Since you are getting fairly consistent 500 +/- then you have probably solved most of your problems.

 

[Gretik]

Cheers for the advice, will leave it cocked for as long as I can - but I'm playing in about 4-5 hours

 

[Gretik]

Ended up needing to trim it slightly, about half a turn in all - brought it down to just below 500fps consistently. Lost a screw for the oversized hand guard, which I need to seriously work on to make secure - but other than that it performed flawlessly!

 

[craftsniper]

Hello, I have got an A&K SVD with a M150 spring, upgaded hop chamber, 650mm 6.03 tb. i will be adding the upgraded trigger sear. I do however have a seal problem as the gun shoots 430fps and I do think that might be a bit to low for the setup I have. Can anyone help me and my SVD?

 

[RR01]

Do you have the original 600mm barrel? If so, what FPS does it shoot at with that fitted? It could be the 6.03 might not be sealing fully in the hop unit. Did you change the hop rubber at the same time? If not, try a new one, but also try sealing it tightly to the inner barrel. Apparently dental floss works well if tied tightly around the front of the hop rubber.

Other than that, it could be the 650mm inner might just be too long for the standard cylinder, cylinder head and piston setup, so dropping your FPS. You could do with upgrading all of the parts on the ASPUK or AirsoftPro.cz websites.

 

[craftsniper]

I do have the barrel that came in the svd, however i have misspaced the small plastic clip that clips onto the barrel.

 

[craftsniper]

I have managed to find the plastic part. I have installed the M150 spring, advanced piston, advanced hop chamber, trigger sear and the barrel the gun came with. It now shoots 525-530fps! I may leave it cocked over night to see if that brings it down below 500 but oher then that I will need a new lower spring.

 

[RR01]

Sounds good now. If it doesn't drop enough then an M140 spring should get you sensibly below the 500 fps limit.

 

[Bullet]

Liittle bit of a necro post but if you are still struggling with an airswal issue try anti blow by rings. Basically a piece of brass that slides over you hop rubber to make the seal stronger. If you pop it into youtube there is a video about them.

Bullet