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Piston Valve 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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Unread postAuthor: chinnerz » Sun Apr 18, 2010 2:52 am

Technician1002 wrote:I have a couple comments.

To get it to stop leaking out the barrel, the sealing surfaces must be true. When the surface looks like a hand carved piece of rubber without a smooth flat surface, the irregularities will provide ample room for air to leak through the low places. Placing the piston in a drill press and dressing the face to be true to the valve seat may help this issue. I have hand lapped some pistons to a valve seat in some of my cannons. Two that I have built with hand lapped piston faces include the Dragon and the Mouse Musket. They are 1 inch and 2.5 inch valve seat PVC piston coaxial cannons.

that is a great point which i didnt think of at first, did you cast that piston? or did you machine/ shape it from a bigger block. since you said you cut it off, you may want to cast your next one.


like that^^
make sure you place it on a flat surface (tape down) and make sure you cut the PVC true. the only issue with hot glue (as you may know) is air bubbles. i have no idea hot to avoid them and they may be a bit of an issue as they ass bumps to you piston face where air can escape. a bit of a tip, if you want to do something with it, like drill a hole or something, of you you are just inserting it into a tight spot, chuck it in the freezer for 20 mins, comes out a lot harder.

personally, after much experience with hot glue (i would say a good 5-10% of my last cannon is hot glue) i would use something else as a sealing face. i used hot glue for my piston and a 2.5mm thick rubber sheet which i cut into a disk for my cannon as hot glue didn't seal for me (low pressure gun).
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Unread postAuthor: Technician1002 » Sun Apr 18, 2010 3:58 am

bobgengeskahn wrote: The O-ring is tight on the piston and, for the most part is just barely larger than the OD on the piston itself when it is on.

What should the tolerance be on the piston in relation to the barrel?

I was talking to a friend and he too recommended lithium grease as well, so I will give that a shot first.

I size the piston to be just loose without any ring on it so it falls freely.

I size the ring to fit the cylinder the piston rides in. It should slightly compress going in. Remember, it seals against the wall.

The space between the piston and cylinder wall should be very small so the O-Ring can't get squished into that space. The diagram in the Wiki shows this relationship well.

This page in the wiki has a great graphic at the very bottom of the page showing the 2 sealing surfaces. They should both be flat and smooth. The seal on the side of the O ring should never be able to slide out of the groove into the space.

http://www.spudfiles.com/spud_wiki/inde ... ston_valve

Note the piston is not rubbing the cylinder. Only the O ring is touching both. The o ring is not down to the bottom of the groove. Only a small amount of compression is needed to squeeze this into the cylinder.
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Unread postAuthor: bobgengeskahn » Wed Apr 21, 2010 1:48 pm

Well, I have some free time this afternoon so I will post an update later if all goes well.

@ Chinnerz, I turned the body of the piston down from a larger OD acrylic rod, and tried casting a face onto it with hot glue.

I spent some time already this morning sanding down the piston so it would fall through the pipe like what Technician described and got some lithium grease to try out, but i have a feeling that the problem will still be the sealing face. But, if all goes well ill post some pics later and hopefully a video if I can get it working!! :D

EDIT: sanded down the piston a bit, opened up the O-Ring groove, re-did the sealing face and added some lithium grease... works like a charm!!!!

pics and video to come later... once i find the camera...

Edit 2: (not sure if double editing is ok... but no one else has replied soooo....)


Improved Piston:


Cant really tell that well, but I took a drill bit and enlarged the O-Ring grove, took down the OD and put a new hot glue face on it. (putting it in the freezer was a great tip!!)

Here is the whole testing set up:
Rigged a 1/4 quick connect to my compressor so I could test it at ~200 PSI before plugging it into CO2

and (because I know that someone will ask), this is the top section, the part coming off to the left is a 3/8 flare nut to 4" 1/4 nipple that I had laying around just to fit it with a temporary barrel. The part off to the right is there so that I can add another piston and a bolt action when I eventually make the actual barrel fitting:

aaaaand here are some videos:

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