A good look at your spark plugs tells how well your engine's dialed in. Magnification with powerful light allows you to zoom in, enlarge and illuminate plug tips for thorough inspection. Reveals oil deposits, signs of detonation and cracked or chipped insulators. 10X magnification provides greater depth of field. Requires 2 "C" type batteries (not included). Use Standard 2.5V, 3A screw-in flashlight bulb for replacement bulb.