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Piston Trouble

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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Piston Trouble

Unread postAuthor: koolaidman » Sat May 24, 2008 9:15 pm

Hey, im not a noob to pneumatics, but this is my first attempt at a piston. It is not working, hence this thread, and im out of ideas. Im making an m203 for the bbmg im making so it's a 2" coaxial with a (realisitc) 40mm bore. Simply put, the piston leaks like crazy. For my first piston i thought it came out pretty well. It is a 1.25" flat top cap with a .25" nut, bolt, and washer and 1-3 neoprene washers and a cork on the back to reduce deadspace and act as a bumper. Ive tried everything, really, it seats very well against the barrel but the air just shoots out from around it. the only thing i can think of now is trying to get a dremel in there an trimming down the sealing face of the barrel so that it's less thick, safety issue? any advice is helpful, just dont tell me how inefficient it is, i didnt design it for max performance. Thanks guys, sorry to make a "noob" thread.
Edit: sorry, pics
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Unread postAuthor: STHORNE » Sat May 24, 2008 9:37 pm

It actually looks very good for your first piston.

Some questions though:
*Are you sure the piston and barrel line up exactly?
*Is the rubber sealing face on the piston reasonably soft?
*How much does it weigh?

Oh, and you can probably use a smaller washer, because if the piston isn't lined up exactly, then the washer will probably get in the way and prevent a seal. But if they do, you could try lubing it up and put some grease (just a tiny bit) along the sides and around the edge of the sealing face.

just my Image
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Unread postAuthor: no-limit » Sat May 24, 2008 9:44 pm

I did the same thing on one of mine, all I changed for it not to work was add the nut, washer and bolt to it. So, the nut and washer might be pushing down on the rubber creating a space like on mine.
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Unread postAuthor: koolaidman » Sat May 24, 2008 9:47 pm

STHORNE wrote:*Are you sure the piston and barrel line up exactly?
*Is the rubber sealing face on the piston reasonably soft?
*How much does it weigh?

Oh, and you can probably use a smaller washer, because if the piston isn't lined up exactly, then the washer will probably get in the way and prevent a seal. But if they do, you could try lubing it up and put some grease (just a tiny bit) along the sides and around the edge of the sealing face.


*They are lined up, just barely. the diameter of the washers are just a mm smaller than the diameter of the 2" pipe, so they have to line up.
*yes, they are indeed neoprene
*it weighs 1.6 ounces (dunno why pistons are measured in ouces...)
i dont understand why Id want a smaller washer?
I did put some vaseline on the sealing part, it helped a little bit, definately noticable, but still not useable.
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Unread postAuthor: MrCrowley » Sat May 24, 2008 9:47 pm

Air might be leaking through the bolt and washer. Happened with me once. The air could be leaking where the sealing face meets the end cap, or the cork.

Edit: What pressures have you tried to get it to seal at and what did you use? Compressor, bike pump, 12volt etc.
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Unread postAuthor: koolaidman » Sat May 24, 2008 10:50 pm

MrCrowley wrote:Air might be leaking through the bolt and washer. Happened with me once. The air could be leaking where the sealing face meets the end cap, or the cork.

Edit: What pressures have you tried to get it to seal at and what did you use? Compressor, bike pump, 12volt etc.


Well, mr crowley, in fact it does leak out of there... as well. I just didnt bother mentioning that because i can remedy it with epoxy. I have tried sealing it with a compresser, a homemade pump, and even my lungs... to no avail.
Does anyone think that trimming the sealing face on the barrel will do anything. id basically make it sharp like you do on the muzzle of an actual gun (to cut the potato.) then thered be a much greater pressure against the neoprene and it would dig in better?

edit: 100 posts! :wav:
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Unread postAuthor: MrCrowley » Sat May 24, 2008 11:04 pm

So it does leak out the bolt/sealing face? Well then that's your problem. It can leak pretty bad too, when I sealed it with epoxy, the leaks went away.
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Unread postAuthor: Biopyro » Sun May 25, 2008 3:29 am

If it leaks around the edges, then I'd say 3 things.
First problem might bethat the washer is too tight, so its pushing down the sealing face and stopping it sealing properly.
Second thing is that the sealing face is too thick. When I made my first piston, If i tried to stick the sealing face to the piston, or make it too thick, for some reason it just leaked.
Lastly, check that the barrel is totally perpendicular to the piston, if they aren't square onto each other, then they'll never seal.
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Unread postAuthor: jrrdw » Sun May 25, 2008 5:54 am

Unless you have a jig that can keep the cutter depth exactly the same all the way around, I'd stay away from using the dremal. The leaky center bolt is probly the problem, I like MrCrowley had the same problem.

I used JB Weld to seal mine, it's made of wood so it worked very well. I have my doughts about the bolt being to tight because my sealing washer looks like a cone mine is so tight, and it seals fine.
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Unread postAuthor: psycix » Sun May 25, 2008 6:50 am

koolaid how much pressure do you use?
Some pistons need just more pressure to seal.
If you are using like 20 psi, try to get it up to 50 psi and see if it still leaks.
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