NGK plugs.

Champion RC12YC's in the avatar. Note the center electrode appears to be missing. Nope - it's there, just pushed up inside. I got these back to Champion and had their tech specialist have a look at them. He explained that today's plugs have a test they have to pass that this ancient design does not. Gas pressure pushes the center electrode up and breaks the seal. This obviously way opens up the gap as well as allowing the gasses to escape between the metal housing and the ceramic. As for the seal?? It's actually compressed talcum powder!!

I caught this at the track when my mph seemed to be off a couple. Pulled the plugs and 2 of them were "surprise, surprise, surprise!" It's now got Autolote AP3924's and seems just fine with those.

I learned something for sure!!
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