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Why Won't My Piston Cannon Work?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Why Won't My Piston Cannon Work?

Unread postAuthor: watto » Fri Oct 19, 2007 8:31 pm

I finished my first piston gun yesterday and I gave it a go this morning but it was a complete failure. When you fill it with air the piston seals the barrel after a while although it leaks air very slowly, when you pull the air gun trigger it just blows air out then about 2 seconds later you hear the piston slowly move back and hit the cap.

I have a few theories, first and most likely there is way too much pilot volume because the plug is hollow, second the blowgun seems to not let air out that quick, after I bought it I noticed there was a sticker hidden underneath it saying 90PSI max, they're not cheap either 8 bucks or something. Or maybe the piston needs an equalization hole, it slides smoothly in the chamber and is a very good fit, it's a coupling that I clamped onto a drill and sanded till it fit in the chamber.

If it's the pilot volume thats the problem maybe I could find something to fill up the space inside the plug and leave holes for the fill valve and blowgun, but youde think if there was too much pilot volume the gun would still fire just not that well. I'll try putting an equalization hole through the piston but if that doesn't work I'll probably give up on piston guns and maybe try another sprinkler valve one, this thing cost me quite a bit too...
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Unread postAuthor: e1337 » Fri Oct 19, 2007 8:38 pm

Your piston might be to large for the chamber, weigh too much, or your pilot may be to small.

-e1337
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Unread postAuthor: mopherman » Fri Oct 19, 2007 8:43 pm

If your piston is any larger than '1, you need a better pilot. try a 1/4 ball valve. good luck
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Unread postAuthor: hi » Fri Oct 19, 2007 8:47 pm

or the piston is not round enough. if you molded it then thats not the problem though. chances are that the piston is to small, not to big. if it was to big you wouldn't have gotten it in the pipe. how much air leaks around it? there should be little space for air (if any) to leak around the piston.

if the piston it AT ALL loose, then go around it with some duct tape. it should be a perfect fit, not loose or tight.



another thing is you said "you hear the piston slowly move back and hit the cap". you do have a bumper dont you? if you dont have a bumper then you cannon will eventually shatter from the force of the piston hitting it. if you dont have one, then its accually a good thing that the piston wont slide back.
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Unread postAuthor: Marco321 » Fri Oct 19, 2007 9:23 pm

If your drawing is to a rough scale, then your problem will be your pilot flow. try a ball valve, if it still doesn't work we know its not the pilot and then will probably need pictures of the inside to determine the problem.
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Unread postAuthor: watto » Fri Oct 19, 2007 9:31 pm

ok what im gunna do is cut the piston in half, so its only about 3.5cm long. then get a 15mm coupler and glue it onto the barrel inside the chamber, thereby compensating for the length taken off the piston. Next I will fill the plug up with hot glue then drill holes for the tire valve and the 1/4inch ball valve which mopherman suggested. That should solve all my problems including the slow air leak into the barrel because the coupler will be perfectly flat. Any suggestions b4 I go to the hardware store?
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Unread postAuthor: Marco321 » Fri Oct 19, 2007 9:36 pm

watto wrote:ok what im gunna do is cut the piston in half, so its only about 3.5cm long. then get a 15mm coupler and glue it onto the barrel inside the chamber, thereby compensating for the length taken off the piston. Next I will fill the plug up with hot glue then drill holes for the tire valve and the 1/4inch ball valve which mopherman suggested. That should solve all my problems including the slow air leak into the barrel because the coupler will be perfectly flat. Any suggestions b4 I go to the hardware store?


I wouldn't do that if i was you. It sounds like you don't get a large enough pressure differential, the last thing you want to do is make the piston heavier so it needs an even larger pressure differential. I would get a 1/4" ball valve, whack that on and see what happens. it should fix your problem.

EDIT: My bad, misunderstood - ill msn ya :P
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 19, 2007 11:13 pm

If your piston is any larger than '1, you need a better pilot. try a 1/4 ball valve.


It depends, if built well you can pilot a 1" piston with a schrader :)

If it's the pilot volume thats the problem maybe I could find something to fill up the space inside the plug and leave holes for the fill valve and blowgun


Please do that, remember, the piston only has to move back 4 milimetres in your case, anything more is just waste!

Or maybe the piston needs an equalization hole


No, that will only make things worse. I would agree with hi that maybe wrapping the piston with tape will help. Also, cutting down the piston is a good idea, it certainly dousn't need to be longer than the diameter for the chamber.

you should have listened to my advice in the first place ;p :D
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Unread postAuthor: MrCrowley » Fri Oct 19, 2007 11:26 pm

Get at least a 3/8" ball valve for a pilot, 1/2" would be better, the bigger the better, it wont waste any air, it will save it if anything.

I had the same problem, I also had a 50mm chamber, I upgraded to 1/2" ball valve which worked great, a blow gun did the same thing to me as with you.

If changing the pilot valve to 1/2" ball valve doesn't work, make sure the piston is tight, but not tight enough so you can't push it with your finger. And make sure you use some lube.

And guys if he's using a 50mm chamber for the piston housing, the piston will be more like 60-65mm
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Unread postAuthor: spudgunnerwryyyyy » Fri Oct 19, 2007 11:56 pm

I completely agree with Mr. Crowley on getter a MUCH bigger pilot. 1/2" works and have a semi tight piston is important. Also using more lube is always better. And contrary to what most people think a bigger pilot won't waste any air if the piston is a good enough seal.
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Unread postAuthor: watto » Sat Oct 20, 2007 4:24 am

I got a 1/4inch ball valve, like one person recommended, I can always upgrade to a bigger one in the future though. The gun is working now but lacks power, but I think filling up the hollow plug should help a lot.

I was testing it before epoxying the valves and didn't notice the tire valve was leaking a tiny bit around the side because the loud little compressor was on, so I shut the boot of my car and left it to fill for a while, then I unplugged the cigarette plug for the compressor then BANG! I opened the boot and found obliterated potato all over the carpet. Luckily I keep a thin sheet of metal on one side of my boot to stop things getting shot through the side of my car LOL. I don't understand how a leak can trigger the valve but my blowgun couldn't.

I'll put up some pics and probably a video when I glue the valves, fill the plug and paint it. Another problem I noticed is that the barrel is bent so I might sleeve a bit of 32mm over the 25x15 reducer right to the end of the barrel and glue a circle of wood inside at the end of it holding the barrel straight, you'll see what I mean. I got this amazingly powerful green laser pointer imported from hong kong that im gunna use for a sight, this thing has a clearly visible beam at night like a light saber that goes on forever. I shone it in the eyes of this stupid crow that tries to kill the finches in my aviary and it took off and flew straight into a tree. Apparently some guy got done for shining one at a plane and blinding a pilot, so they must be able to go pretty far.


jackssmirkingrevenge wrote:
If it's the pilot volume thats the problem maybe I could find something to fill up the space inside the plug and leave holes for the fill valve and blowgun

Please do that, remember, the piston only has to move back 4 milimetres in your case, anything more is just waste!


I wanted to get a cap but they only had hollow plugs, I'm going to fill it up with hot glue and put straws or something similar to leave holes for the fill and exhaust valves.

I was aiming to make the piston only move back 4mm but because the plug wont go all the way in its probably more like 1.5-2cm. Will this make the valve open any slower or will it just be a waste of air?

Thanks to mopherman, mrcrowley, jacksmirkingrevenge and especially marco who stopped me from cutting the piston in half and drilling a hole in it on msn.
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Last edited by watto on Sat Oct 20, 2007 4:35 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 20, 2007 4:34 am

Will this make the valve open any slower or will it just be a waste of air?


Since there is more air in the pilot chamber, the pressure will drop slower for a given pilot valve and therefore the piston will start to move slower.
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Unread postAuthor: watto » Sat Oct 20, 2007 4:43 am

jackssmirkingrevenge wrote:
Will this make the valve open any slower or will it just be a waste of air?


Since there is more air in the pilot chamber, the pressure will drop slower for a given pilot valve and therefore the piston will start to move slower.


So in theory it will make it open slower, do you think the difference will be noticeable enough to warrant me extending the barrel by gluing a shortened coupler onto the end inside the chamber bringing the piston closer by 1cm or so?
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 20, 2007 4:51 am

That will save you almost 20cm<sup>3</sup> of pilot volume, go for it ;)
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Unread postAuthor: MrCrowley » Sat Oct 20, 2007 4:56 am

watto wrote:I got a 1/4inch ball valve, like one person recommended, I can always upgrade to a bigger one in the future though. The gun is working now but lacks power, but I think filling up the hollow plug should help a lot.


I doubt it will be much, I usually fiddle around with pilot volume measurements on GGDT and an increase of 100cm^3 gives no change in velocity most of the time for my SGA gun(depending on cannon).

I had a 3/8" ball valve on it once, which has a QC on the end, I could only get about 200m with a golfball, now I get 400 with a 1/2" ball valve, which is now upgraded to a 1" sprinkler.

I strongly recommend a 1/2" ball valve, it'd do a lot more then filling the plug, and it will save air, which saves pressure and gives more power.
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