Features: Working Principle: This spark plug test pen determines the quality of the coil according to the spark color and flash distance. Unique Design: There is a visual window through which you can see the length of the spark. The spark resistance is 10K, 20K, 30K and 40K ohms, and the white font makes it easier for you to read the gap measurement. Come with a adjustable screw, you can adjust the spark gap by turning it. ( Note:A good and flawless ignition system should lead a gap of at least 15mm.) Powerful Function: The In-line spark plug tester can test spark gap and check the power service. This spark tester can be used for small engines and some motorcycles because the standard plug wire can be unscrewed. Widely Applications: This Ignition System Coil Test Tool can apply for 2 cycle small engines,cars, motorcycles, boats, chainsaws,string trimmers, lawn mowers more. Easy to Operate: Ground the clip to the engine. Insert the other end into the spark plug wire. Read the length of the spark through a small window. The longer the spark gap, the stronger the coil.