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Two inch piston not re-seating

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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Two inch piston not re-seating

Unread postAuthor: iisthemuffin » Mon Nov 03, 2008 8:08 pm

So ive been working on a two inch coaxial cannon as some of you may know.

It fires very well but it wont reseat. Its kind of weird.

If i take off the back and i move the piston, when i go to pump, it will seal. But im not pushing the piston up to the barrel when i move it. No matter how i move it, it seals. I can pull it all the way back and screw on the back and it fires.

After the first shot i go to repump but air comes from the barrel. So i open up the back and i try to push the piston foward. Its already all the way against the barrel. So i put the back on and it shoots.

I went through this so many times trying different things to make it re seat. I even modified my piston about 7 times and the problem continues.

So basically, i can get one shot from my cannon and then i have to remove the back for it to fire again.

If you have any ideas i would appreciate them greatly. This is frustrating the crap out of me.
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Unread postAuthor: kablooie » Mon Nov 03, 2008 8:16 pm

Is your sealing face being sucked partially into the barrel and getting stuck there? That would explain why when you move it it will seal, because the sealing face would be freed from the barrel and would pop down into position. That's all I can think of for now.
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Unread postAuthor: iisthemuffin » Mon Nov 03, 2008 8:34 pm

If it were sucked into the barrel wouldnt it be sealed?? And if it were being sucked in wouldnt it be sucked in the first time? So it wouldnt pilot the first time right??

Im not trying to be rude or anything, but please read everything i have said, and think aobut your response before replying.

And i am not being ungratful either. I appreciate any help anyone offers. But i dont feel as though you read my entire post.
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Unread postAuthor: kablooie » Mon Nov 03, 2008 9:42 pm

I did read your entire post, what I mean (and this happens occasionally on my 3" piston cannon) is that when you fire the gun, the sealing face edges could be getting pulled into the barrel by the rush of air leaving the cannon. With the sealing face out of place at least partially inside the barrel (but still attached to the piston by a bolt or whatever), when you pump it up for the second shot it the air would simply escape without the rubber sealing face properly located to create the seal.

I remedy this problem on my cannon by pushing the piston backwards until the sealing face edges pop back to there proper position, allowing me to take another shot. Also, it only occurs for me on low pressure shots when the piston does not move back far enough to pull the sealing face out on its own.

I hope this makes sense, and I realize my description might not be the best, so if you don't understand, I can make up a diagram.
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Unread postAuthor: iisthemuffin » Mon Nov 03, 2008 11:31 pm

Sorry for being so snippy earlier. But what you are saying actually does make sense to me. Im not home now but when i do go home i will try pushing the piston back after i shoot it and see if that helps.

Thanks for your idea.

Edit: I was just thinking. How could the edge of my sealing face get inside? That actually doesnt make sense. My chamber is 2 inches and my barrel is 1 inch. I have a 3/4inch washer on my sealing face. The edge of my sealing face never gets close to the barrel. It would have to fold to get in there wrong because my piston is too long to turn sideways.
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Unread postAuthor: jook13 » Mon Nov 03, 2008 11:51 pm

Edit: I was just thinking. How could the edge of my sealing face get inside? That actually doesnt make sense. My chamber is 2 inches and my barrel is 1 inch. I have a 3/4inch washer on my sealing face. The edge of my sealing face never gets close to the barrel. It would have to fold to get in there wrong because my piston is too long to turn sideways.


Maybe it is folding? His explanation really is the only one that I can think of. Perhaps look into it before dismissing it as impossible.
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Unread postAuthor: Gippeto » Tue Nov 04, 2008 1:12 am

With a 1" barrel, and 3/4" washer, I can't see the sealing face getting sucked into the barrel IF the total piston travel is anywhere near the 1/4" to 3/8" it should be.

If you shoot it, then without trying to pump it, take the back off, where is the piston coming to rest?

Is it touching the barrel? Or is it near the back?

How much of your barrel is inside your chamber?

Do you have anything keeping the barrel centered? (near the piston)

It may be helpful if you would conduct a little test and post the results.

Colour the washer (and only the washer) with something which will rub off easily. (wax crayon, marker, etc.)

Assemble, fire a shot, and the carefully remove the piston. Are there any areas on the washer where the colour has rubbed off?
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Unread postAuthor: psycix » Tue Nov 04, 2008 2:29 pm

Pictures may help here.
Have you tried pumping/adding air faster? More pressure may always help, even though it leaks on the first few pumps.

The weirdest thing is that it does seal after you opened and closed the back.
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Unread postAuthor: Fnord » Tue Nov 04, 2008 6:58 pm

I can say almost for certain your sealing face is getting sucked in. I had this problem for a long time on my hybrid, with a 2" sealing face, 1" barrel and 3/4" washer holding it back.

Try super gluing your sealing face to the piston (leave the bolt in, though).
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Unread postAuthor: iisthemuffin » Tue Nov 04, 2008 10:06 pm

Thank you everyone. And once again, i apologize kablooie. I think it is getting sucked in and i was firing today and trying what you said. After each shot, i would push the piston back with a ram rod and everything went fine.

And i will trying try super glueing the rest of the face down as suggested. Thanks.
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Unread postAuthor: kablooie » Wed Nov 05, 2008 12:37 am

No worries, I'm glad to hear your cannon is on it's way to repair.
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