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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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Unread postAuthor: DYI » Wed Jul 27, 2011 10:15 am

The only true NPT sparkplugs I know of are those that the Model T (and probably some of its contemporaries) used, which have a 1/2" NPT thread. They're somewhat oversized for small projects though, and tend to fail at around 2500-3000psi based on my testing.

...the flash was almost deafening.


What size of cap are you using with this flash circuit? :lol:
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Unread postAuthor: saefroch » Wed Jul 27, 2011 11:07 am

I just put two photoflash capacitors together in parallel. I know it's nothing compared to what you do, but it was pretty darn loud.
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Unread postAuthor: DYI » Thu Jul 28, 2011 8:28 pm

Ah, good. I was worried you might be playing with an ignition system that would directly initiate detonation of the mix :P
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Unread postAuthor: Mr.Tallahassee » Thu Jul 28, 2011 9:45 pm

If you want to use a long reach spark plug you can drill the threads and re-tap the hole. 1/4 NPT fits a spark plug hole almost perfectly. That's how I made my Ghetto Compression Tester. :lol:
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Unread postAuthor: saefroch » Fri Jul 29, 2011 9:19 am

I ground down the threads on a spark plug and began epoxying it in, but I stopped with it only slightly attached because I'm not sure if I can see to check if it's arcing when I take the tee that it's epoxied into off. I'll take care of that within the next few days when I get a BBQ ignition system.

DYI wrote:Ah, good. I was worried you might be playing with an ignition system that would directly initiate detonation of the mix :p
Oh come on, you know I'm not yet capable of that :wink:
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Unread postAuthor: saefroch » Sat Jul 30, 2011 5:19 pm

Final additions to the piston are curing now, which means I'm adding the impact-resistant head to the piston that will contact the barrel to push the piston pack. The rear seal is only composed of the end of a bolt with copious amounts of teflon tape around the threads, I'm hoping to maintain some amount of serviceability, since the silicone o-ring on the internal piston will probably be toasted after a few high-mix shots.

If the epoxy cures correctly and the parts stay aligned, I should be able to fix the final pinhole leak and test this as a pneumatic tomorrow.

EDIT: Meat thermometer says my toaster oven only goes as low as 156F :(
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Unread postAuthor: saefroch » Sat Jul 30, 2011 6:33 pm

In all its ghetto glory: Image
It's... mostly finished. Is it possible that the 5W-30 I use is causing something to swell?
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Unread postAuthor: POLAND_SPUD » Sat Jul 30, 2011 6:42 pm

Is it possible that the 5W-30 I use is causing something to swell?
maybe
however as '5W-30' referrs to viscosity of it only we need more info
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Unread postAuthor: saefroch » Sat Jul 30, 2011 6:56 pm

Is this good enough?
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Unread postAuthor: jackssmirkingrevenge » Sat Jul 30, 2011 11:34 pm

saefroch wrote:Is it possible that the 5W-30 I use is causing something to swell?


Most likely.

Use molybdenum grease.
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Unread postAuthor: saefroch » Sun Jul 31, 2011 10:57 am

Molybdenum grease? Yeah... I'll look around for it, but will the classic plumber's silicone grease also avoid swelling?
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Unread postAuthor: SpudBlaster15 » Sun Jul 31, 2011 5:08 pm

It's generally not a good idea to use petroleum oils/greases on rubber parts, as it can cause them to swell. The 'Molybdenum' grease suggest by JSR is most likely to be a petroleum based grease with a small percentage of molybdenum disulfide added as an extreme pressure lubricant, so I would advise against it.

Silicone grease is what you want. It won't affect the seals, and it tends to be quite resistant to high temperatures/direct flame exposure. The plumber's grade product is actually a very pure and high quality material - It's used on faucet washers in drinking water supply systems, so it must be free of any contaminates that could pose a health hazard.
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Unread postAuthor: saefroch » Sun Jul 31, 2011 7:51 pm

I think I saw some last time I was at a local hardware store. Thanks, I'll be sure to get some.
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Unread postAuthor: Technician1002 » Sun Jul 31, 2011 8:02 pm

saefroch wrote:EDIT: Meat thermometer says my toaster oven only goes as low as 156F :(


Use a light dimmer to run the power way down. The thermostat won't function, but you can really reduce the heat. Put a light bulb in parallel to get a visual indication of power output.
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Unread postAuthor: saefroch » Sun Jul 31, 2011 8:32 pm

Or I could just stick it in the oven at "warm" which is usually 120F.

Spark plug epoxy is curing right now. I don't think I got any epoxy on the electrodes, but even if I did, I should be able to sand it off. Tomorrow I'll be cleaning the oil out, greasing, and pressure-testing everything.
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