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Spark Plug Threads

Dual plug engine?

Take an old plug, break out the insulator, glue/solder/weld in a coupling for an air line.

Blow air into one plug hole while chasing the other plug hole. Valves should be closed.

This is guaranteed to get crud in your eye------wear goggles.
 
I second the use of bearing grease on the tap, but if you have BoeLube, that works even better. I would combine that with the idea of blowing air into the other plug hole as Mike said. If you don't feel like welding, I'd recommend using a compression tester and plug low pressure air onto the fitting.

Knowing that you'll be looking down the hole while crud is coming out, I would not use more than 30 psi, and I second the use of safety glasses plus a face shield.
 
Slickest solution I found -- search Amazon for "back tap". You put a coat of grease on the threads, slide it INSIDE the head, expand the tap to engage the inner threads, then back it out. Do that a couple of times. It will chase the threads from the inside out. All the chips end up outside the head, AND you don't have to worry about starting the tap crooked. I bought the 12mm size to correct a slightly buggered up plug hole on my Rotax, and it works great. Well worth every penny I spent on it.

And no compressed air required.
 
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