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How can I make my piston seal against the barrel?

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 can I make my piston seal against the barrel?

Unread postAuthor: nitornos » Tue Nov 04, 2008 11:42 pm

First of all, hello spudfiles, this is my first post, and I wanna thank everyone on here for inspiration and all their awesome s***.
I've got my first cannon coming along. It's pretty aspiring for a first cannon. I don't know much spudding lingo, like what porting is, but it has a 3" chamber a total of 5 feet long and a 2.5" barrel 2-3 feet long, and a piston in a 3" tee. the piston is a segment of 2.5" pipe that fits loosely in the 3" pipe. The valve is supposed to be piloted by a 1/2" ball valve. Ace hardware doesn't sell sprinkler valves. :( I put 7mm thick red gasket rubber as my sealing face. Apparently, the piston won't seal against the barrel.
I have some possible problems already, but I'm kind of fed up with making this gun. It's been a real hassle. So I want to know just what to pay the most attention to.
1. Could my piston not be long enough, or not fit snugly enough inside the tee, causing it to seal in a slightly angled manner? How much space should be between the piston and housing?
2. Could I be using the wrong kind of sealing face? Does it have to be neoprene?
3. or must the barrel be smoothly sanded, with no imperfections or deep scratches on the area that seals against the piston, and of course, perfectly square and level?

One of my friends helping me build was a dumbtruck and glued my barrel into my tee when I wasn't looking, so I don't know how to check the barrel for squareness. I can still reach into the pilot end, but what I was thinking was I could custom make a bushing out of cast epoxy and then epoxy that into the barrel, that would take care of that problem. Does there have to be a sealing face on the both the piston and the area it seals against, or is only the piston enough?

Thanks for any help anyone can offer, it will be greatly appreciated.
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Unread postAuthor: jook13 » Wed Nov 05, 2008 12:42 am

Did I read that right? 3 inch chamber of 5 feet long and a 2.5 barrel of 2-3 feet long? Unless that is a typo, you need a longer barrel my friend.

Are you sure ace doesnt sell sprinklers? the ace where I live has tons of them. They are not in the plumbing section though, they are in the gardening section where they sell fertilizer and hoses as well as sprinklers and the valves that go with them.

anyways, I will try and answer what I can...

1. The most important issue I have found is to make sure that the piston and barrel are cut square. Not the shape, I am talking about a perfect 90 degree angle cut. You want the piston to be just tight enough to slide when you push it with your finger but not take much effort to do so.

2. the red rubber gasket rubber should be fine. make sure the sealing face is clean and free of debree.

3. I think I answered this with 1 and 2.

a few questions..

Where are you filling the chamber up from?
Have you tried lube to help the piston slide easier?
Have and photos or diagrams of the gun?
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Unread postAuthor: kablooie » Wed Nov 05, 2008 12:44 am

Just from what you've written, I would say the major issue is the fit of your piston inside the tee, the piston should fit snugly, while still being able to slide when pushed with a finger. There could be other problems as well, if possible some pictures of your valve and piston would help us figure out the issue(s).

As for the barrel imperfections/unevenness, one solution would be to glue duct tape to the front of your piston or something of similar shape and put it into your valve. Then if you spun the piston (with it pushed up against the barrel) the sandpaper should do it's magic. The success of this depends on how far off your barrel cut was, if it is too uneven you'd have a lot of sanding to do.
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Unread postAuthor: nitornos » Wed Nov 05, 2008 4:43 pm

@Jook, I guess the barrel is a little short, but I wanted it to be easily carried. I made it for my dick cheney halloween costume, but I couldn't get it to fire so I just took the shell with no innards. I'll put a longer barrel on later.
The piston is loose enough to bang around when I shake the gun. I guess that's a little too much?
Not many precautions were taken to make sure the cuts were very square, that must be my main problem. If kablooie means what I think he does, I'll try his idea.
As for pics, the barrel is already glued into the piston, I wanted to keep it pushed in for Halloween but not glued. So the only pictures I could take would be down the back of the hole where the piston goes, and my camera needs new batteries anyway.
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Unread postAuthor: iisthemuffin » Wed Nov 05, 2008 8:33 pm

I have been told before "if the piston is such a tight fit you couldnt imagine any air possbily getting past it, its just right"

With that in mind, remember that, like others said, you should still be able to push it with your finger with a little bit of resistance.

The barrel should be completely square or it will not seal, as you have noticed.

I have a question though. How exactly did you deal with the fact that the pipe is hollow? If air is going through your piston and into your barrel, well thats your problem.
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Unread postAuthor: nitornos » Wed Nov 05, 2008 11:10 pm

I crammed 6 1/2 inch segments REALLY tightly into it, and glued gasket rubber onto the ends with epoxy. it's airtight, I checked it in a bowl of water, and no bubbles came out.
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Unread postAuthor: iisthemuffin » Thu Nov 06, 2008 2:57 pm

Alright. if its as airtight as you say then your problem is probably the barrel not being square.
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Unread postAuthor: dongfang » Wed Nov 12, 2008 4:53 pm

Hi,

If your valve is made serviceable, so you can take the piston out through the back, you can save it: Make a second piston with a flat front (maybe the original will work too), and glue (water soluble maybe) some good quality sand paper on the front of it. Attach a shaft of some kind that will stick out the back. You can use that as a tool (press against barrel end and turn) to sand the barrel end square to the piston axis.

I used that trick once, and it worked great. Even though I had cur everything perfectly square, there was a nasty surprise somewhere. Maybe I had not glued in the barrel straight, or the tee, for some reason, was not made quite straight.

Regards
Soren
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