Piston valve sensitivity

[FighterAce]

So.. I was working on this piston valve for the last few weeks with some pretty good results. The gun fires nicely with the ball valve as the pilot but when I try to replace it with a schrader it doesnt work at all. My question is: how can I make it more sensitive so it can be piloted with a schrader? To me it seems like I made all the criteria... The piston fits very close to the housing walls, theres minimum space behind the piston to the schrader/end cap and I've made the piston head bigger then the barrel. Pls help I have no idea what to do next...

Heres a few pics to show you guys how the whole thing looks like
http://img710.imageshack.us/img710/7156/p2040111.jpg -the whole set up with the ball valve as pilot
http://img525.imageshack.us/img525/3174/p2040112.jpg -muzzle end, front of the piston
http://img208.imageshack.us/img208/1927/p2040116.jpg -view from the ball valve - pilot
http://img5.imageshack.us/img5/1428/p2040117b.jpg       -piston made from a piece of pvc filled with hot glue
http://img46.imageshack.us/img46/6261/p2040118.jpg      -piston in the 3/4 connector
http://img34.imageshack.us/img34/9127/p2040119.jpg      -insides of the 3/4 connector with no piston
http://img46.imageshack.us/img46/2083/p2040120.jpg      -my metal schrader valve attached to the 3/4 end cap
http://img294.imageshack.us/img294/8160/p2040121.jpg   -schrader with the end cap
http://img188.imageshack.us/img188/2395/p2040122.jpg   -back side of the piston inside the 3/4 connector
http://img99.imageshack.us/img99/8545/p2040125.jpg       -piston all the way back
http://img341.imageshack.us/img341/5985/p2040126.jpg    -and front
http://img35.imageshack.us/img35/6707/p2040128.jpg       -replaced the ball valve with the schrader valve and flipped the air tank
http://img211.imageshack.us/img211/4227/p2040132.jpg   -3/4 to 1/2 reducer and garden hose adapter as the barrel sealing face
http://img697.imageshack.us/img697/9369/p2040134.jpg   -view of the front of the piston without the reducer and adapter inside the 3/4 T

And some technical data... If you need anything else pls ask
Piston sealing face diameter: 25mm
Piston body diameter: 20mm
Piston length: 50mm
Barrel diameter(part that seals the piston): 13mm
Gap between the piston and barrel: 7mm
Pressure: 10Bar

I know its all scratched up   I've been trying to get this working for a long time...
In case you're wondering why I have those 2 valves under the T..  all the parts below the T (in the setup where the ball valve is the pilot) are actually for my M4 so those 2 are my firing and safety valves. The wood is not done yet so I'm using its new air tank for experimenting with piston valves. I'll probably have to replace those metal pipes... they are in some really bad shape  

[Forum Admin]

What type of lubrication are you using? Sometimes a bit of heavy grease on your piston is all it needs to make an effective enough seal to pilot with a schrader.

[FighterAce]

Just mechanical oil. I've been looking for grease but I can only get a whole bucket.... no small package 

[Forum Admin]

try some petroleum jelly, it will do the same trick. Man, a bucket of grease would last a lifetime and then some. 

[FighterAce]

try some petroleum jelly, it will do the same trick. Man, a bucket of grease would last a lifetime and then some. 

I'll get that bucket tomorrow... Then I'll let ya know if it started working. I hope it does..
Btw. will I still be able to pump air in trough that same schrader if the grease makes a better seal?

[Forum Admin]

You'll still be able to pump the air in, though it will probably offer more resistance.

[FighterAce]

EDIT
I deleted my whole comment because I hit jackpot 
I added a spring in the front of the piston to help keep it open if it decides to bounce back and I made a small round slot round the middle of the piston to act like a grease storage slit and prevent it from gathering on the edges. And it worked. When I depress the schrader it opens instantly with brute force I couldnt even imagine with bv. Thank you so much for suggesting grease 
I might even make a vid this weekend to demonstrate it

[Forum Admin]

Good to hear that worked for you, grease can definitely make up for not being able to machine parts to close tolerances.  I hope you do make a video on it, I would like to see how it works for you.

[FighterAce]

I just took some test shots and I can tell ya... I'm impressed
The shot on the pic was just 10 bar and the 7cm nail dart went 4cm into the wood. When I took it up to 13 bar, the dart exploded  Time to start reinforcing my bullets hehe



With paper removed...


btw. the thickness of the wood is 2.5cm

[Forum Admin]

very good results.  Man, your nails look a lot better quality than the crappy ones they sell here at Walmart. lol

[FighterAce]

very good results.  Man, your nails look a lot better quality than the crappy ones they sell here at Walmart. lol

Yep they are very bad nails but good bullets. A good nail should have a blunt tip so the wood doesnt split when you drive a nail in, blunt tip pushes material away and reduces the possibility of a wood split. And these are very sharp and yes high quality. Thats why I buy those   

btw. the piston I'm using had been sanded down to make it fit but now I check if a piece of the same material would fit inside and it did. The problem seems to be some bump at the end of the reducer. If I replace the piston body I'd have much closer tolerances and it would probably be more reliable. I'd also eliminate the need for a grease sink along the way 

[BoyntonStu]

very good results.  Man, your nails look a lot better quality than the crappy ones they sell here at Walmart. lol

I bought 2 batches from WalMart.

The first was OK.

The second was outstanding!

Shiny, a little thicker, well made nails.

Lowest bidder.

[Forum Admin]

My 1st batch was pretty decent, this last batch is only 75% ok. Not very uniform length / weight. I have found a place I can buy better looking ones even cheaper by the pound, so when I use up all these (5 pounds) I'll probably go there. They have some bigger nails I'd like to give a try as well.

[FighterAce]

I finished the M4 so I had to get a new air tank and schrader for the piston valve. Here it is..


I made the piston seal so good that when I pump air behind it, it doesnt leak air into the air tank so I have to use the other schrader. I started making a little equalization hole but its still not enough. I'm removing 1 tenth of a millimeter at a time just to be sure I dont over do it. I was thinking to make a triggersystem to depress the schrader on my new gun so I'll need another schrader to fill it anyways.

I was having major problems with the piston. When I greased it, after a few shots all the grease would go out the schrader and the gun wouldnt work anymore. Then I made a new piston with a slit for an O ring.


After a few test shots I noticed it lacked speed. When I depressed the schrader, there was this 1 second of silence and then it would go off. Searching for a solution I noticed I made the 2nd piston too long and the air didnt have much space to flow through when the valve was opened. I just took away 5mm at the back and it started working great. Here are pics with the piston in the most forward and back position.




There is quite a lot of snow around here so I thought why not use it. I've placed a 100cm long barrel with a 40mm bore onto the muzzle and shoved a snow slug into it. It went about 100-150 meters. I was suprized what this valve could do with such a small air tank.

I was planning my next piston gun but got stuck on the safety part... How big is the chance of ADs with piston guns?  Would a trigger block be enough to keep the gun from misfiring or do I need something to block the air too?

[Forum Admin]

There are two times I've ever had an accidental discharge with a piston valve. The first is if there is a leak on the pilot side of the valve. With your o-ring design, it wont take much of a pressure differential to get it to fire, so be sure you seal the pilot side up well.  Since you are piloting with a schrader, a safety can be something as simple as a little bar that blocks it from being pressed. The other is when the sealing face fails. This isnt as serious as it doesnt produce as powerful a shot, but it can still be dangerous. Your sealing face looks good and strong, so you shouldnt have too much to worry about there.