And it just hit me,...instead of machining a bushing, I don't see why you could not use non-fouling adapters. These are the bandaid-fix deals that you thread into the plug hole and then thread the plug into that. They have a small hole in the combustion end that keeps the oil leaking past worn out valve guide seals from fouling the tip, but allows the intake charge to still reach the plug. I'd just take a pair and grind away the restricted end until enough of the plug is exposed. Then drill the proper sized hole in the pipe, insert adapter, weld, and it's good to go.
I used to build propane/oxygen machine gun simulators. I know a few things about installing spark plugs in places they were never intended to go and making them do things they were never intended for.
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Well according to the specs, the spark plug has to be 8-10 inches from the end of the pipe or you won't get proper flames. With most cars, that 8-10 inches is underneath the car and not visible. My 5 foot exhaust stacks are nothing BUT visible. It would look pretty shitty if I had a hole drilled way up there and then a wire trailing down, so I can't have the wire outside the pipe.
Even if I welded a mount for the spark plug inside the pipe, I still would have to find a way to hide the spark plug wire. The only way to hide it is to have it run inside the pipe, which is why I thought up that idea. And if the wire is inside the pipe, it will melt where it touches the pipe; hence the need for a heat-resistant sheath.
I might be wrong, but if you put them lower, at the bottom of the vertical pipe, they still might work. You'd ignite the gas, and it would blow up, out the top, reaching the air, then it would fully ignite in the air.
Your original plan is good dave
There are industrial compression fittings available I dont think you need to use spark plug wire you could use heavy gauge electrical
Wire there wont be any flame until after the plug check around with a local industrial electrician the only concern would be EGT