The Thexton Manufacturing THX404 Adjustable Ignition Spark Tester works on ignition systems from high energy electronic automotive systems to small engines. This tool tests for no-start conditions and for spark strength. An adjustable gap allows for spark gap requirements from no volts to 40,000V. Gauge background makes the spark easy to see for accurate measurement. Alligator clip securely fastens to ground. Screw assembly adjusts spark gap quickly and easily.