I was too harried, when my classic BMW motorcycle died on the side of a busy highway, 150 miles from home, to visually test both the plugs. This simple tester would have saved me from my own laziness.

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http://feeds.boingboing.net/~r/boingboing/iBag/~3/53qdRTTO_II/now-i-carry-an-in-line-spark-p.html