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Piston problem

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 problem

Unread postAuthor: spud yeti » Mon Jul 02, 2007 4:28 am

Hey, I have just finished my coaxial piston valve cannon and have a problem. The piston wont seal against the barrel and air is flowing into the barrel. My piston has a neoprene disk on it too. Is this because too much air is leaking around the piston? Otherwise what is it? Please help! Thanks!
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Re: Piston problem

Unread postAuthor: jackssmirkingrevenge » Mon Jul 02, 2007 4:51 am

spud yeti wrote:Is this because too much air is leaking around the piston? Otherwise what is it? Please help!


No, it means that either the piston face or the end of the barrel isn't at 90 degrees to the travel of the piston.
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Unread postAuthor: jrrdw » Mon Jul 02, 2007 4:51 am

If you can remove your piston, pull it back out and look at where it's been making contact with the barrel. If you can't tell, smear some greese on the barrel sealing face, put it back together and try again with only enough pressure to make it leak just a little bit, and pull the piston back out and look again. That should let you know exactly what the problem is.
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Unread postAuthor: spud yeti » Mon Jul 02, 2007 4:57 am

Ah thanks, I'll just go do that now and give an update soon! Its highly possible the edge isnt 100% square come to think of it... that sucks! :(

EDIT: (about 10min later) You were right, the neoprene disk had a lump in it, so I cut it down and now Im waiting for the glue to dry!
Once again, thanks a lot for answering the stupid question! :D

EDIT: i did that and now it seems to seal all the way round, but it leaks a very little bit... is this a problem?? Or how can you get 100% flat neoprene?
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Unread postAuthor: MrCrowley » Mon Jul 02, 2007 5:45 am

When I had that problem the bolt didn't seal against the washer and neoprene as my piston was semi-hollow the air just flowed through the gap between washer and bolt. I sealed that up and now its 100% AOK
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Unread postAuthor: spud yeti » Mon Jul 02, 2007 6:20 am

OK cool, I'll check that too then. hopefully that will otally sort the problem out totally!
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Unread postAuthor: jrrdw » Mon Jul 02, 2007 6:29 am

That reminds me, i used JB Weld around the center bolt that holds the rubber to my piston, i had the same leak as you MrCrowley.
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Unread postAuthor: spud yeti » Mon Jul 02, 2007 6:31 am

OK, so do you just smear some around the bolt/washer?
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Unread postAuthor: jrrdw » Mon Jul 02, 2007 6:35 am

I packed the hole about 1/4 the way and then screwed the bolt through. That way it stayed solid/no air bubbles.
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Unread postAuthor: spud yeti » Mon Jul 02, 2007 6:37 am

OK cool, thanks a lot. I'll go do that now I think. give you an update just now :wink:
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Unread postAuthor: jackssmirkingrevenge » Mon Jul 02, 2007 6:50 am

remember that as you up the pressure, the harder the neoprene presses against the barrel and the better it seals, so a small leak at say 15 psi might disappear at 100 psi.
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Unread postAuthor: spud yeti » Mon Jul 02, 2007 6:57 am

Yeah, as I blow harder against the piston, it seals more and more, so if I used a compressor not a bike pump, it wouldnt be a problem i guess. Thanks for the help!

EDIT: woohoo! I have no leaks now! Thanks a lot guys!
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Unread postAuthor: Modderxtrordanare » Mon Jul 02, 2007 2:19 pm

spud yeti wrote:Yeah, as I blow harder against the piston, it seals more and more, so if I used a compressor not a bike pump, it wouldnt be a problem i guess. Thanks for the help!

EDIT: woohoo! I have no leaks now! Thanks a lot guys!


When you blow, you're only producing about 2-3psi if I recall correctly. But if you had a compressor, then you'd have a constant stream of airflow, as compared to pumping it with a bike pump.
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Unread postAuthor: spud yeti » Mon Jul 02, 2007 3:09 pm

yeah, im pretty bleek at the moment, because i tried pumping my gun up, but the piston wouldnt seal. But on the bright side, im going to the store tomorrow and getting a compressor fitting for it and then I'll try it on the compressor :D
What do you guys do if youre just using a bike pump, because mine isnt working too great!? :wink:
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