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Piston Not sealing.

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 Not sealing.

Unread postAuthor: SpoonZ » Sat Jul 07, 2007 4:07 am

Im having a problem with my first piston. its a barrel sealing T valve. 50mm bore, 32mm port. For some reason the piston doesnt seem to seal against the barrel.

Im filling with a schrader. Im using a lynx can as my piston. It fits with not even .5 of mm to play with. So equalization holes arent needed. The sealing face is neoprene. its thick. around 3mm or more.

Do I need an air compresser? Do I need to 100% seal the back of the piston?

I just dont know why its not working. Im filling from the pilot end of the T.

Just throw out your ideas why it isnt working.

THanks.
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Unread postAuthor: frankrede » Sat Jul 07, 2007 4:26 am

Do you have a spring behind the piston?
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Unread postAuthor: MrCrowley » Sat Jul 07, 2007 4:46 am

You don't need a spring. Do you have a bolt to hold the face on? If so JB-Weld(or other alternative) the bolt up. I had the same problem, air going through the piston and out the bolt, I JB-Welded it up and its 100% fine now.

this isn't JB weld, its marine epoxy but it still worked till I dropped it:
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Unread postAuthor: SpoonZ » Sat Jul 07, 2007 6:40 am

But its 1/4 filled with hot glue. That should seal it?

If I push the piston hard enough against the barrel a friend isnt able the blow into and make it leak. Yes I know this isnt enought pressure. But one pump shouldnt be too much higher. So I dont know. The piston isnt pushing against the barrel hard enough. If I packed heaps of foam type stuff that I use as a bumper/end of the piston it would most probably seal.

the top only has one neprone disk. the same size as the lynx/axe can.

I think the piston might be too heavty. As its pretty heavy atm. But I have no idea. THis is my first.
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Unread postAuthor: MrCrowley » Sat Jul 07, 2007 7:25 am

Mine was filed with hot glue to. I remleted over the bolt on the inside and epoxied the bolt. When blowing you don't notice it leaking, trust me I tried.
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Unread postAuthor: SpoonZ » Sat Jul 07, 2007 7:34 am

The piston weighs 200grams, its pretty heavy. I just dont think theres enough force of air to push the piston against the barrel.

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Its sealed at the bottom. And has a special packing type of foam for a bumper.
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Unread postAuthor: Infernal Maveric » Sat Jul 07, 2007 12:00 pm

Me think's the problem is that there isn't enough force comming from the scharder to seal the piston against the barrel.

I had this problem with my gun, so I added a spring and it worked fine, until the spring weakened -.-

Have fun muching about with springs and such.
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Unread postAuthor: VH_man » Sat Jul 07, 2007 12:43 pm

is that funky thing that looks like a (hot dog) your sealing face or your bumper? if its your sealing face, thats your problem.....

hwever, that did give me an idea. has anone tried making a cone-shaped sealing face? it would proboably work way better than a flat one.

anyway, try a spring. it should work.

OOOHHH did you lube your piston at all? cover the thing in WD-40...............

i know my first pison wouldnt seal at first too. then i dipped the thing in sillicone jelly and re-inserted it, and WOHOOOOO loudest noise i have ever heard...........
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Unread postAuthor: MrCrowley » Sat Jul 07, 2007 5:54 pm

You should clear some of that JB weld up it will get in the way of the seal, notice how mine is alot more compact. If the JB hardens on the sealing face it will leak.
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Unread postAuthor: SpoonZ » Sat Jul 07, 2007 9:45 pm

I only just put the epoxy on. And I havent tried it. Ill see what happens now. Then if not Ill start to take the piston apart and retry.
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Unread postAuthor: SpoonZ » Sun Jul 08, 2007 8:37 am

Haha, I got the piston to seal. But the piston is too heavy or the compression spring is too hard to compress. As it had to have a heavy load of force to push it. Ill have to down grade to a smaller one.
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Unread postAuthor: MrCrowley » Sun Jul 08, 2007 9:03 pm

Ok so what are you piloting the valve with? With my piston it wouldn't pilot with a 3/8"(10mm) ball valve but pilots perfectly with a nice gun shot crack with a 1/2" ball valve. So try a 1/2" ball valve to pilot it.
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Unread postAuthor: SpoonZ » Mon Jul 09, 2007 3:06 am

I have a 15mm ball valve but atm Im piloting with a blow gun. I dont know how Im going to thread 25mm of poly nipple into the back of the T valve.
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