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Bad piston, please help!

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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Bad piston, please help!

Unread postAuthor: cyanite » Mon May 07, 2007 7:01 am

OK, shortly after building my piston gun... I realized the piston was very bad, and was causing major issues with the rest of the gun.

So far:
-The bumpers have come off, and got lodged INSIDE the sprinkler valve, almost launching out the other end...
-The piston randomly decides whether to seal or not
-Piston occasionally gets jammed in a random location
-Sprinkler valve has stopped working, it leaks air at the rate I pump it back in
-I'm not too sure if A) the piston is actually equalizing B) the foam got destroyed by the barrel ramming into it several times

So... this is obviously a coax set-up, the chamber is 2" wide, and the barrel is 1/2". As far as I can tell, the actual gun itself is immaculate, the piston is causing the problems.

The piston is constructed from half a deodorant can, wrapped in tape, smeared heavily in 'BP Energrease" (A lithium based, high pressure lubricant).
The bumpers and barrel-sealing are made from some sort of roll of air-tight soft foam-like material I found... probably was some sort of roll out camping mattress.

Anyways... here is a diagram of the piston and barrel I put together, I will get some actual pictures of it in the near future. It is color coded, the yellow is PVC, green is metal, red is glue, and blue is the foamy stuff.
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Unread postAuthor: jrrdw » Mon May 07, 2007 7:14 am

Change you piston all together, concave/dome style piston's allways seem to be problematic, use something flat with rubber for the barrel seal, not foam. Add a spring behind your piston, then a piece of wooden dowel to protect the sprinkler valve and to adjust the spring tention. My piston still gets jamed from time to time, i use a long bolt to unjam it, just unscrew the sprinkler valve, stick the long bolt in and move the piston forward, put it back together and shoot some more.
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Unread postAuthor: cyanite » Mon May 07, 2007 7:19 am

Hmm, any ideas on what to use... I need something that will fit into the chamber nicely, and be easily made from household objects... would rubber off the sole of a shoe work... I'm all out of other stuff to use. Any other rubber I have available is either too hard (like rock hard) or too porous. Any plans on a ghetto or cheap to buy/make piston for a 2" (53.5mm) would be much appreciated... Nothing that needs excessive machining either, as only pretty basic tools are available
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Unread postAuthor: Mr.Russ » Mon May 07, 2007 7:23 am

maybe use a tennis ball, cut it to fit in the barrel, have the inside surface seal against the barrel. used the other half as a bumper, gut a 1" hole in it for the opening of the sprinkler. have the inside part of it facing the sprinker.

im not sure
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Unread postAuthor: jrrdw » Mon May 07, 2007 7:46 am

I used wood for my piston, cut it out of a 2X4 with a 2 1/2 inch hole saw, then sanded to fit 2" pipe, with 2 o-rings and a 1/8th equilazation hole hidded under the barrel seal, barrel seal acts as a check valve.
Cutting out the 1st piece -- Image

Fitted to pipe -- Image

Piston inside my valve -- Image

Easy to make.

In action -- Click here.
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Unread postAuthor: cyanite » Mon May 07, 2007 7:51 am

That's pretty nice.. I gotta go to the hardware tommorow anyway to get 2 threaded sections (for piston servicing >>>$27<<<!!!!!!!) so I might as well pick up some wood and O-rings... Ill go google check valves now... seems useful for use in a piston
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Unread postAuthor: jrrdw » Mon May 07, 2007 7:16 pm

cyanite wrote:That's pretty nice.. I gotta go to the hardware tommorow anyway to get 2 threaded sections (for piston servicing >>>$27<<<!!!!!!!) so I might as well pick up some wood and O-rings... Ill go google check valves now... seems useful for use in a piston


Drill the equilization hole off-set of center, but DO NOT drill a hole through the rubber seal. The equilization hole has to reside outside the barrel, inside your valve, or it wont work. If you drill it inside the barrel raidis, the building pressure will just slowly close the equilization hole.
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