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How tight should my piston be?

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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How tight should my piston be?

Unread postAuthor: iisthemuffin » Sun Jul 20, 2008 2:03 am

Im trying to make a mini coaxial piston gun. The piston is pretty much a perfect fit in my opinion. There is just a tiny gap bettwen it in and the pvc. But when i try to pilot the piston i just empty out my pipe and nothing happens. I havent tried orings but i could. Is my piston not piloting because the pipe is so little and all the air rushes out before the piston slams back? Its 6 inches by half inch by the way.
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Unread postAuthor: starman » Sun Jul 20, 2008 2:12 am

Is this the mini you were working on last night? Is the piston sealing? Can you pressurize the chamber?
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Unread postAuthor: Gippeto » Sun Jul 20, 2008 2:18 am

I suppose the "rule of thumb" should be;

When your piston fits so closely that you can't imagine the chamber filling,... then it's just about right. :wink:

Too loose of tolerance combined with a pilot valve of too little flow will yield the results you mention.

Try to eliminate as much of the "gap" around the piston as possible.
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Unread postAuthor: iisthemuffin » Sun Jul 20, 2008 2:19 am

Yes it is the same mini. Well not the same one. Same design, different parts.

I had posted in the other thread that i made a mistake and the piston was not servicable so i had to get different fittings and rebuild.

And yes i got it sealing PERFECTLY. Im not sure if you have ever seen them. But my dad had this things to move furniture. They are white octagons with this really thick rubber padding on them and it works wonders. Its very soft so the barrel is able to push into it, But its still hard enough to withstand the pressure.

I can pressurize but i have a tiny leak somewhere. I had troubles finding a fitting to put in my barrel. I used some compressor fittings. Works nice. Come to think of it, do you think that tiny leak is why i cant pilot?
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This is essentially what we've got.
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Unread postAuthor: starman » Sun Jul 20, 2008 2:29 am

No can usually still pilot if you're leaking a little. Are you sure your sealing face isn't leaking?

As Gippeto said, chances are good that your piston is too loose in the cylinder.

Why didn't you just post this problem on last nights thread?
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Last edited by starman on Sun Jul 20, 2008 2:30 am, edited 1 time in total.
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Unread postAuthor: iisthemuffin » Sun Jul 20, 2008 2:30 am

Im relatively positive. Ill go pump it and test it again.

The piston is so close of a fit. Its just got like 1/32 of an inch between it and the pipe.
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Unread postAuthor: starman » Sun Jul 20, 2008 2:34 am

iisthemuffin wrote:Im relatively positive. Ill go pump it and test it again.

The piston is so close of a fit. Its just got like 1/32 of an inch between it and the pipe.


Oh that's way too much, especially for your small piston. It needs to actually be just touching but still able to slide freely. You should be able to pull a little air vacuum when you move it back and forth...hear it burp when you pull it out, etc.
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Unread postAuthor: iisthemuffin » Sun Jul 20, 2008 2:36 am

Oh wow that tight? Even without o rings in should burp?
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Unread postAuthor: starman » Sun Jul 20, 2008 2:42 am

iisthemuffin wrote:Oh wow that tight? Even without o rings in should burp?


Yep without O rings, and not a highly pressurized loud burp, just feel a little resistance and hear some air rushing. When you get that, your piston should work. With your small piston, the resistance and burping may not be as apparent as it is with larger ones.

This was what I was telling you last night. Your challenge now is to make a piston work in there. Be patient with it, don't get discouraged. You will come to a solution eventually... :wink:
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Last edited by starman on Sun Jul 20, 2008 2:49 am, edited 2 times in total.
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Unread postAuthor: iisthemuffin » Sun Jul 20, 2008 2:44 am

Wow i was actually thinking this one may be too tight. Hmm should that burp occur from the sides of the sealing face or from the actually piston material?
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Unread postAuthor: starman » Sun Jul 20, 2008 2:51 am

iisthemuffin wrote:Wow i was actually thinking this one may be too tight. Hmm should that burp occur from the sides of the sealing face or from the actually piston material?


From just moving it back and forth in the cylinder...yours may be too small to have much effect like that. Just make it fit VERY well.
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Unread postAuthor: iisthemuffin » Sun Jul 20, 2008 2:53 am

Is that with or without the sealing face?


I tihnk ive got what you spoke of. It doesnt move on its own but i does slide easily if i push on it. Good?

Im assuming some vaseline greasing is in some order to make it slide easily?


If you hadent noticed. Each spaced out piece is an edit after i think of a new thing.


Well when i put the piston in, and i suck on the barrel it shoots up to seal it and if i blow it blows back down the pipe. It will go back and forth with ease. Im going to go try without greasing and see what happens.


Oh. And if your wondering. The material i decided on for my piston, well more like was so lucky to find, was a crayola marker. Too bad my hands are completely purple now....


Gah! I cant test because the "small" whole i had is larger than i had initially thought. I tried to epoxy some copper tubing into a brass hose barb and its leaking. I guess im going to mix up some more epoxy and give it another go. I hope it work.s I want to do this so bad man. :cry:


Wow. That was a terrible epoxy job. I think i had too much hardener and not enough resin. As soon as i started twisted the hose barb the epxoy started to fall right off.

Well its all epoxied up. Hopefully that my only leak. Im out of epoxy. What i have is a half inch to one fourth inch adapter. The hose barb is screwed into that and then i slide my copper tubing into the hole of the hose barb. I find this effective in keeping the tube straight. I thought i had it fixed but it was leaking from where the tubing was inside the hose bard. So i covered the part of the tube that goes in the barb with epxoy and slid it in. I made sure to put excess so it makes a good epoxy on the inside and underside. Then i epoxied around the tip of it. I hope it holds.

Im sorry im writing so much. Im using this kind of to update everyone so that you can help as much as possible if i need it.

I hope to hear from you starmman since you have been my primary source in this project. Thanks for the help thus far.
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Unread postAuthor: starman » Sun Jul 20, 2008 12:04 pm

Actually some pictures might be helpful at this point. You're confusing me a little, moving from an well fitting piston problem to a hose barb epoxy repair adventure.

So, you have changed pistons...the crayola marker and you are awaiting an epoxy repair to test?
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Unread postAuthor: iisthemuffin » Sun Jul 20, 2008 12:06 pm

Haha yea. I know its confusing. Im just kind of running into problems and fixing them as i go. I will get some pictures in just a sec.

Just to clear things up. I switched my piston, to the crayola marker, for a tighter fit. I went to test it and i had a pretty bad leak at my hose barb so i epoxied that last night and let it dry over night. And thats where im at right now.

Will edit with some pics in just a moment.

Just read the part where you had asked while i didnt post this in the original thread. Im not sure actually. I guess that had never corssed my mind. I will be sure to remember that for next time so i dont clutter things up with more threads.

Epoxy repair escapade was a success. I can pressurize with no leaks. NONE. It hols air steady without dropping.

But i tihnk my piston seals too well. Like the copper tubing pushes too far into the rubber pad and it gets stuck and wont pilot....
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DSC00691.JPG
New crayola piston.
DSC00691.JPG (34.43 KiB) Viewed 434 times
DSC00692.JPG
Outside of hose barb.
DSC00693.JPG
Inside of adapter.
DSC00693.JPG (32.34 KiB) Viewed 434 times
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Unread postAuthor: starman » Sun Jul 20, 2008 12:48 pm

Macro mode on the camera please...the little flower. Your pics are out of focus on the things you want to show.

It was suggested earlier that you use real rubber seals and not mouse pad material. I again offer that suggestion for the sealing face...mouse pad foam will be fine for the bumper.
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