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Hybrid Design & Spark Plug Questions

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Hybrid Design & Spark Plug Questions

Unread postAuthor: Pete Zaria » Wed Jul 18, 2007 3:01 pm

I've had my sights set on a hybrid for a long time now, and have never had the time/funding to build one (I probably still don't, but it's looking more and more realistic, so I'm designing one).

I had originally wanted to build a golfball hybrid, but realized that the cost of doing so out of metal would be more than I'm willing to spend (and I'm not willing to run more than 2x mixes in any plastic pipe, either). So I've decided on a marble/ball bearing "sniper" style hybrid.

So, I'm hoping to build a small, all-steel, pressure rated, hybrid with no welding required (meaning threaded fittings, seeing as I don't have a MIG/TIG rig and I'm pretty sure my crappy old arc welder won't do the job).

I'll be taking a lot of design cues primarily from spudblaster15's gun (and from several other guns posted here), since it's similar to what I want to build, although I'm considering going gen 2, and I'll be using more gauges (to attempt to measure peak pressure, etc...), a remote ignition system, hopefully an on board air system (fire extinguisher and regulator?), and some other goodies.

I considered tinkering with a no-burst-disk design, but decided to get my feet wet in the hybrid territory (though I believe I fully understand the concepts...) before building anything with a decent chance of mangling me. I like to participate actively in the community and help to push forward the design and theory of new concepts, but I think I should make sure I know what I'm doing before I go trying to win any design competitions.

Anyway, I'll be posting drawings, ideas, and questions about this project in this thread. Hopefully.

Onto my questions....

I'm hoping to find a threaded adapter that will allow me to screw a spark plug into a T on the chamber.

The good news is, any type of spark plug could be modified to work, so I probably have a large array of sizes to work with - 4 stroke model airplane plugs, chainsaw plugs, lawnmower plugs, of course car and motorcycle plugs, etc...

My first question is, does anyone know if there are any type of spark plugs that use NPT threads? That would make this extraordinarily easy.

Second, does anyone know what the common thread size/type is for automotive spark plugs?

Third, has anyone seen this done before, know if it's possible/where to get the parts, or have a better idea?

Fourth, if all else fails, I suppose I could drill press a hole in an end plug, tap it, insert the spark plug, and use an extravagant amount of JBWeld on both the inside and outside of the spark plug, and then test it remotely A LOT....

Thanks in advance for the info.

Peace,
Pete Zaria.
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Unread postAuthor: spanerman » Wed Jul 18, 2007 3:22 pm

the spark plugs ive used are m10x1.00 mm and m14x1.50mm hope that helps i can get the spark plug numbers for these if you wish.
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Unread postAuthor: CannonBall » Wed Jul 18, 2007 3:54 pm

its m14x 1.25 thats the original tap. for cars and mopeds it what u find the most for sparkplugs
You wont find a fitting that fit a sparkplug ive done the research already 8)
look at this picture. done with an arc welder is sealed and strong and works really nice..
Or look at my hybrid at showcase.
Fitting is normal malleable steel so an arc welder is good enough but dont make it to hot...
A small tip.. weld it in a fitting sealed 100%. .... and very strong there's almost no way to blow it out of the fitting trust me ive tryd it with another sparkplug and a fitting ive put a charge of 1 gram AP.. acetonperoxide under it and not much happend.

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Last edited by CannonBall on Wed Jul 18, 2007 4:00 pm, edited 1 time in total.
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Unread postAuthor: spanerman » Wed Jul 18, 2007 3:55 pm

hmm unuasual....maybe i mesured wrong...
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Unread postAuthor: spudthug » Wed Jul 18, 2007 4:10 pm

i screwed my sparkplugs into my chamber without jb weldeing them and they havent blown out..why would they??
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Hybrid- 75% done. need to build propane holder and drill/tap sparkplug hole..
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Unread postAuthor: spanerman » Wed Jul 18, 2007 4:12 pm

they wouldent.....but its a bit extra security... what mixes have you gone to ST?
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Unread postAuthor: CannonBall » Wed Jul 18, 2007 4:17 pm

well aparantly we have different thoughts but no problem o would go for welding if not tapt well a spark plug is a deadly projectyle.
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Unread postAuthor: frankrede » Wed Jul 18, 2007 4:36 pm

My old autotech teacher had a spark plug tap kit, we used it on engines when some dumbass managed to f* up the threads
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Unread postAuthor: spanerman » Wed Jul 18, 2007 4:43 pm

they are chasing taps..good for fixing damged threads....not for cutting new ones, i had to buy a spark plug...then mesure the threads, and then buy a tap(expensive size $20) then whack it in...then JB weld for good messure..
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Unread postAuthor: SpudBlaster15 » Wed Jul 18, 2007 6:08 pm

This is what I used as a spark generator in my hybrid. I couldn't find a spark plug tap, so I made a spark plug that uses 1/8" NPT threads.

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Unread postAuthor: spanerman » Wed Jul 18, 2007 6:18 pm

nice idea, it could be threaded into a fitting...no need to drill/weaken chambers. how well does it hold pressure?
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Unread postAuthor: SpudBlaster15 » Wed Jul 18, 2007 6:25 pm

I tapped 1/8" NPT threads directly into the center of my chamber, as installing it in a fitting at the rear of the chamber would decrease the burn rate of the fuel and thus decrease performance. I don't think it reduces the integrity of the chamber at all.

It has held up well to numerous 4x and 5x firings.
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Unread postAuthor: spanerman » Wed Jul 18, 2007 6:35 pm

yeah, i doubt a 1/8 reducws strenth much, if at all....mine has a 9mm hole in it....im definatly remote fireing the first few shots...
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