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More 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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More piston problems

Unread postAuthor: Arborman » Thu Jun 07, 2007 12:21 pm

I have tried to get this gun to work for about two weeks now and ...

i got nothing.

so i need some help.

it is a 4" coaxial with a 2" barrel.

When I fill it up (50 psi) the piston seals fine. When I pull the blowgun, it doesnt shoot the stuff i put in there and i think it all comes out through the pilot valve. the piston is a sanded 3" copling with a plywood disk with some rubber glued on.

http://www.spudfiles.com/forums/files/pilot_168.jpg

http://www.spudfiles.com/forums/files/piston_154.jpg
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Unread postAuthor: Modderxtrordanare » Thu Jun 07, 2007 12:28 pm

Sounds to me like your piston is jammed and/or permanently stuck.

I don't know if your piston is servicable or not, but I made one that wasn't and I put the barrel in to far and it was ruined because the piston couldn't move.
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Unread postAuthor: Arborman » Thu Jun 07, 2007 12:31 pm

The piston is servicable. I seriously do not think that it is jammed, i have taken is out and messed with is a thousand times.
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Unread postAuthor: Modderxtrordanare » Thu Jun 07, 2007 12:44 pm

How tight of a fit is the piston in it's housing?
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Unread postAuthor: judgment_arms » Thu Jun 07, 2007 12:46 pm

Okay, I’ve had to overcome this problem a lot.
First and for most, with the thing completely, save for the pilot valve, use a rod and seat the valve from the pilot end, then stick a rod down the barrel, measure were the valve face is. Then push on the piston with the rod until the piston won’t move anymore, the measure that. The piston should put up a little resistance, but not too much.
Measure the distance between the two marks, if memory serves, for a two inch barrel your piston should move about an inch.

If that checks out green then,
With the pilot valve still removed, blow into the pilot area, your piston should seat and when you stop blowing the piston should jump slightly, you’ll be able to hear it.

If that checks out green then,
It’s most likely your pilot valve isn’t opening.

If all three check out green then I don’t know what to tell you…
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Unread postAuthor: Hotwired » Thu Jun 07, 2007 12:48 pm

How is the piston constructed?

Can air for example escape back through the piston too easily?
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Unread postAuthor: Arborman » Thu Jun 07, 2007 12:55 pm

uh oh, that may be my problem. crap. i guess my piston sucks.

anyone want to point me in the right direction to make a better one?
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Unread postAuthor: Modderxtrordanare » Thu Jun 07, 2007 12:59 pm

Arborman wrote:uh oh, that may be my problem. crap. i guess my piston sucks.

anyone want to point me in the right direction to make a better one?


Probably, that's why I asked how well it fit inside it's housing. How big is the gap all around the piston? You could give it a few wraps in duct tape to fill in the gap and try that. That's what I did, but with masking tape.
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Unread postAuthor: judgment_arms » Thu Jun 07, 2007 1:08 pm

I recommend electrical tape, on the last few wraps pull it tight and stretch it a lot.

If you decide to redo it I recommend you cast it from hot-glue that’s what I do and it works great.
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Unread postAuthor: lukemc » Thu Jun 07, 2007 2:24 pm

if you have a lathe make some o ring grooves and o ring it up
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Unread postAuthor: Modderxtrordanare » Thu Jun 07, 2007 2:44 pm

lukemc wrote:if you have a lathe make some o ring grooves and o ring it up


Or a table saw, a drillpress/vise, or a hand drill/vise. :wink:
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Unread postAuthor: frocksie » Thu Jun 07, 2007 3:09 pm

This may not work for a 3" piston due to weight, but PVC rod is a great piston material for smaller diameters.
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Unread postAuthor: iPaintball » Thu Jun 07, 2007 3:12 pm

You could just use a 3" encap for a piston so you don't have pilot problems. Mine has one, and it works like a charm.
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Unread postAuthor: theRIAA » Sat Jun 09, 2007 10:15 pm

ive made a 2" piston by melting hot glue sticks in a microwave, pouring that into a greased 2" piece, hot gluing on a piece of rubber, and drilling 2, 1/16" pilot holes.
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Unread postAuthor: wblteen » Sun Jun 10, 2007 3:00 pm

as said by Da Pneu Kid a 3 inch end cap fits perfectly withing 4 inch pipe. Mine also uses this as a piston on my 4 inch coax.
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