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Piston Problems

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 Problems

Unread postAuthor: FordGtMan » Wed Apr 02, 2008 5:39 pm

Well, i got eager with myself after getting some casting epoxy from the craft store, and decided to make my 6 mm mini coax. everything went as planned, JSR's epoxy technique worked amazingly btw :D, but now after everything is dried, I didnt think i needed the darn piston serviceable. The problem is when i pump the thing, the piston will seal and everything, but when i take the pump head off, most of the air comes out of the presta i am using to pilot it. It doesn't seem like to much air, but the thing is made out of 8 inches of 1/2 inch pvc... with my bike pump, i get half a stroke down and it is already pumped to 130 Psi!!! Is this wrong or something? I feel like the equalization isn't happening, even though the piston wasnt airtight. it is just a dowel with electrical tape around it and inner tube to seal it, all held together by a roofing nail... None of the air comes out of the barrel when i fire it, even though there is still pressure cause air rushes out of the presta. any thoughts?
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Unread postAuthor: pat123 » Wed Apr 02, 2008 5:41 pm

Have you tried shooting it with the pump still attached? maybe all of the air is coming out when you unattach the pump
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Unread postAuthor: FordGtMan » Wed Apr 02, 2008 5:43 pm

Is that even possible? wow, that was a quick response! :D
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Unread postAuthor: pat123 » Wed Apr 02, 2008 5:49 pm

yeah it is possible if it is that small. how much room is between the piston and the schrader or whatever you are using?
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Unread postAuthor: FordGtMan » Wed Apr 02, 2008 5:54 pm

There is only about 1/4 of an inch between the barrel and the piston, and the presta is pretty much the same as a schrader, except they have to be on my dads bike for some reason :D

Edit: I just tried letting the gun alone for a minute after pumping it to 120 psi to let the pressure equalize and it fired all the sudden. after each shot, for some reason, i have to push the piston back for the gun to start to seal again. Now every time i am pumping it, it goes off automatically and randomly. ???
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Unread postAuthor: keep_it_real » Wed Apr 02, 2008 6:48 pm

My guess is the piston is actually not airtight enough. It's not going to make a seal with electrical tape if it slides easily in the pipe so it's definitely filling the chamber. But if there is too much flow around the piston, the air will just rush around it.

I couldn't tell from your post but if you can get at the piston, put some more tape on it and try it again.

As for it firing randomly, does it fire? Does that mean it works now?
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Unread postAuthor: Counterstriker » Wed Apr 02, 2008 6:49 pm

Maybe the piston is failing? It happened to me like 100 times.. does yours resemble the one in my sig?
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Unread postAuthor: FordGtMan » Thu Apr 03, 2008 5:59 pm

As for it firing randomly, does it fire? Does that mean it works now?


It does indeed fire now, but it isn't a powerful shot. it makes a "Phut" sound, and really isn't that powerful at all. i loaded a spitball in it and was weak as crap.

I plan on cutting it in half and removing the piston and doing whatever to it. then i will put a tee and an endcap to fill in the space
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Unread postAuthor: FordGtMan » Thu Apr 03, 2008 11:31 pm

Sorry for the double, but as for my piston, i am making something a little more reliable(EPOXY piston :D ) and i have a few questions. As for the piston, should the rubber be completely glued on the whole surface cause on my other one, 3 layers of bike tubes were used, all held by a roofing nail. this means the rest of the rubber was loose, and i think that that was the problem, and could have made the "Phut" sound. And How tight should it be? tight enough that you cant blow the piston out of the tube or what?
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Unread postAuthor: jackssmirkingrevenge » Fri Apr 04, 2008 12:38 am

the rubber should ideally be glued uniformly to the piston face, or if it's the by a nail or bolt, then the diameter of the bolt head or washer should be very close to the inner diameter of the barrel.
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