There are many people running plugs smaller than 1/4 inch. I can't even count how many are running #10-40 plugs. Anybody who has built a little demon is running #8-40 through a distributor. I have sold more than a few #6-56 plugs and have received links to videos of engines running on them. They are like any other plug. If you burn oil they will foul out faster than if in a clean cylinder. If in a clean cylinder you will get many hours of service.
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