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(Kinda)New Spudgunner

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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(Kinda)New Spudgunner

Unread postAuthor: Enferno8 » Sun Jul 31, 2011 10:45 pm

Hey everyone, I'm new here and have a few questions I couldn't find answers to right off hand. So. My newest setup is a 7.5 ft. MAPP gas monster. It's a 12" chamber with a U-Turn into a 7' X 1.5" barrel. I decided to go with a flyback transformer ignition circuit for reliability and I wanted to have two strong spark gaps. Anyway, on to my questions. I'm having some leak problems and was wondering if anyone had suggestions for a high-temp sealant that I could use on PVC. The coupling from the U-Turn to the barrel suffered moderate damage to the threads and now I'm planning to use plumbers putty(basically goopy, non-hardening PVC) to help seal the threads and prevent more heat related leaking damage. Thanks for any suggestions. This thing shoots russets 250 to 300 yards, my furthest yet :D
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Last edited by Enferno8 on Thu Aug 04, 2011 7:16 pm, edited 1 time in total.

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Unread postAuthor: Gun Freak » Sun Jul 31, 2011 10:50 pm

Is it leaking on threads? Are you using Teflon tape? Welcome aboard!
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Unread postAuthor: Enferno8 » Sun Jul 31, 2011 11:00 pm

Well... I actually didn't screw in the barrel in all the way before I secured my hose clamps :? . So what happened was that the threads weren't tight enough to make a good seal. This allowed the hot gas to get through and cause heat damage to the threads. And no, no teflon here. It'd be almost useless b/c of the high burn temp of MAPP.
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Unread postAuthor: Gun Freak » Sun Jul 31, 2011 11:08 pm

Something is not right here. Why did you use clamps? For an atmospheric combustion you don't need to even seal the barrel threads. There is only pressure for a split second, not long enough for significant leakage.
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Unread postAuthor: Enferno8 » Sun Jul 31, 2011 11:40 pm

The hose clamps were just part of my build. As far as the damage went, the gap in the threads (b/c I didn't screw them in tight) allowed some of the hot gas to rush past the threads. The MAPP burns at about 5000 °F, give or take 750 °F depending on the exact fuel/air mix. So after 3 dozen spuds, this little leak allowed the threads to be slowly deformed by those many split seconds of intense heat rushing by the threads. The reason this happens is because of the longer barrel that requires a longer burn to achieve maximum exit velocity.
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Unread postAuthor: Technician1002 » Sun Jul 31, 2011 11:46 pm

Teflon tape will work for sealing that. The exposed edge of the tape may get singed, but with no leakage through the threads, it won't burn through. The PVC in the sealed area will keep the tape in the threads cool.
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Unread postAuthor: Enferno8 » Sun Jul 31, 2011 11:49 pm

The problem is that the damage is severe enough now that just teflon won't be enough to completely seal the larger gap... I'll try to get pics of the whole gun and the damages up tomorrow sometime.
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Unread postAuthor: jsefcik » Mon Aug 01, 2011 12:54 am

Jb weld the holes up
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Unread postAuthor: Crna Legija » Mon Aug 01, 2011 1:28 am

jsefcik wrote:Jb weld the holes up


That would stop him undoing the barrel and who wants to ram spud 7ft down a barrel lol.

You could try cutting a grove for O-ring on the end of the fitting that or cut the fitting off and buy a new on to replace it. Maybe you could put epoxy putty on the area that's damaged and let it harden then with a hacksaw blade connect the left over threads back together but that would take lots of time and probably wont last.
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Unread postAuthor: Mr.Tallahassee » Mon Aug 01, 2011 1:29 am

Actually, use Teflon pipe dope, usually called joint compound, in the threads. You brush it on and it seals as good as Teflon tape and resists heat far better. It's all I use anymore. This is the exact same stuff I use currently. Any brand will do. It also reseals when you take off the barrel for quite a long time before needing to be re-coated. Image
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Unread postAuthor: Enferno8 » Mon Aug 01, 2011 9:19 am

Thanks Tallahassee, that's the answer I was lookin for. I did a little searching for something like that a never really found a better answer than plumbers putty, which isn't great against heat.
Thanks again!
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Unread postAuthor: saefroch » Mon Aug 01, 2011 9:31 am

Personally... I'd just load it up with teflon tape. The issue that you run into with the putty style thread sealants is that over any significant period of time they can behave like epoxy.

Teflon tape is great against heat, its used in hybrids.
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Unread postAuthor: Gun Freak » Mon Aug 01, 2011 11:11 am

Replace the fitting on the barrel and wrap it with teflon tape.

'Nuff said.
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Unread postAuthor: Enferno8 » Tue Aug 02, 2011 7:06 pm

I decided to go with the pipe dope b/c of the fact that both threaded pieces are damaged to the point that i would need a LOT of teflon tape to fill the gap.
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Unread postAuthor: saefroch » Tue Aug 02, 2011 7:32 pm

Does it seal now?
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