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Leaky gun

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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Leaky gun

Unread postAuthor: papa_smurf » Sun Feb 18, 2007 7:34 pm

I recently finished building a pneumatic marble gun using a 1/8" threaded pressure gauge. I drilled the appropriate size hole into my air tank and tapped it accordingly. I covered the threads with plumber's tape and screwed it in. It is easy to turn and when I pressurize my cannon air is leaking through the hole I screwed it in to. Any help on how to seal the hole would be much appreciated. I tried applying epoxy around it and it still leaks.

Thank you.
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Unread postAuthor: LGM » Sun Feb 18, 2007 7:43 pm

It still leaks around epoxy? Did you unscrew it and then apply epoxy to the hole and gauge?
Try silicone sealant of some sort.
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Unread postAuthor: sjog » Sun Feb 18, 2007 7:44 pm

Adapt from 1/8" to 1/4" on the gauge.
Drill and tap again.
If you have to use epoxy it's loose and wrong.
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Unread postAuthor: papa_smurf » Sun Feb 18, 2007 9:14 pm

Thanks for the advise.
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Last edited by papa_smurf on Mon Feb 19, 2007 2:10 am, edited 1 time in total.

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Unread postAuthor: sjog » Sun Feb 18, 2007 9:51 pm

Hey some times you need 4-5 layers of teflon tape and pipe dope.
But if there is any question re-tap larger.
Did you use the right size drill?
When tapping PVC the drill can be slightly smaller but not over size.
Good luck. I don't want your gauge in flight.
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Unread postAuthor: trae08 » Sun Feb 18, 2007 10:17 pm

if thats not working plug the hole with a metal plug using alot of teflon an drill another hole a lil smaller.
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Unread postAuthor: noname » Mon Feb 19, 2007 12:33 am

I'd recommend taking the guage out, clearing all the teflon tape away, and globbing silicone sealant all over the guage threads and screwing it back in. Doing this has stopped over 10 leaks on threads for me.
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Unread postAuthor: papa_smurf » Mon Feb 19, 2007 2:09 am

Well now, I don't think I can remove the gauge because I put epoxy putty around it to try to seal it. I'm not sure if it worked or not, because I need a new valve to be able to pressurize it. I'm crossing my fingers and I don't know when I can buy a new valve, so I'm really nervous. :roll:
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Unread postAuthor: noname » Mon Feb 19, 2007 11:08 am

Putting epoxy around the valve to make it seal has a very low chance of working, don't freak out when it still leaks. I'd cut it off now, and save myself the trouble, but whatever, it's your gun.
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Unread postAuthor: squeaks » Mon Feb 19, 2007 12:26 pm

okay, save yourself loads of trouble in the future and buy yourself a 1/4" and 1/8" npt tap. They work lightyears better than just drilling and screwing the piece in.
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Unread postAuthor: badkarma » Sun Feb 25, 2007 9:54 pm

How come the measurement mentioned (1/4" and 1/8" npt tap) are smaller on my drill bit chart, yet its larger on the actual item (gauge, pressure relief valve, schrader valve)? A 1/4" drill bit is size of a sharpie point, but the gauge with 1/4" thread is wide as a penny. So what am I missing here?

HELP!! :D
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Unread postAuthor: noname » Sun Feb 25, 2007 11:24 pm

Pipe sizes are weird, and I have no idea why the tap sizes are a lot bigger than drill bits, but I do know that pipe sizes go by the ID, so a 1/8" threaded steel pipe should have a 1/8" ID, except it's actually about 1/4." I just go with it, I don't care why it's like that.
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Unread postAuthor: leeeeshad » Mon Feb 26, 2007 6:28 am

tap sizes are bigger than the drill because you have to leave some metal after you drill for the tap to cut, try bike tyre sealant if its a small leek, all you do is put some in the chamber then pump and pump until the leak seals
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Unread postAuthor: papa_smurf » Tue Feb 27, 2007 4:25 pm

The epoxy worked :wink:
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