GearWrench 3305D Compression Tester This Versatile Kit Provides a Quick Method to Obtain Cylinder Compression Pressure on Engines With Deep, Recessed Spark Plugs and Difficult Access A Special Air-Hold Valve is Included for Easy Resetting of the Gauge, Which is Graduated From 10 to 300 PSI (0 to 21Kg/Cm2) Gauge Assembly Includes Pressure Relief Valve Caution: Do Not Use With Engine Running for Intended Use Only