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piston wont close

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 wont close

Unread postAuthor: bl74 » Mon Aug 03, 2009 10:35 am

Ok my gun is just about done. The valve is a 3" piston in a 3" tee with a 2" outlet. My first piston was made out of a food can with tape around it and it worked ok but pressure leaked out the barrel. When i took it out it was all bent up after just 3 shots so i decided to make a new one out of hot glue. I cut some 3" pipe to the size that i wanted the piston to be and poured in hot glue and let it dry. I put on the neoprene, a bumper, lubed it up with some vaseline and it works fine theres no leaks except it wont close by itself when pumping up the gun. When the glue was drying it kinda shrunk leaving a small space around the side but it still fits pretty snug. I tried wrapping it in teflon tape but it didnt work and most of it got ripped up. The new piston is pretty heavy (0.8 lbs) compared to the last one so that might have to do with it but i dont know what to do now.
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Unread postAuthor: Mr.Sandman » Mon Aug 03, 2009 10:39 am

Make one out of pvc. Hot glue pistons are generally used for smaller cannons. Pictures would also help as they can show us if you did something else wrong.
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Yeah, it's that important.
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 03, 2009 10:39 am

You can either wrap it in more durable masking or duct tape to compensate for the gap or fit a small spring between the piston and the rear of the chamber that will also serve as a bumper.
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Unread postAuthor: ralphd » Mon Aug 03, 2009 12:12 pm

Could the piston be getting pinched on the bumper end? I recently changed my pilot valve to a bigger diameter and noticed that I had to bump the launcher's barrel on the ground to "unstick" the piston. When I took it apart to give it a look see I noticed pieces of bumper and a small piece of my piston. It turned out that the bumper sometimes just clears the lip of the new fitting that I used and the lip hits the piston. I glued in a small piece of 1 1/2 then sleaved it with another piece. Now the bumper uses most of its face. The only problem that I am experiencing is the sealing face is no longer airtight. i used lithium grease as someone suggested but after that it started to leak. I'll try the vaseline.
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