A handy little tool that helps you detect the compression stroke of your engine. Just remove one of the spark plug from the #1 cylinder, insert this tool, then rotate the prop and listen for the whistle. That your indicator that the piston is on the upstroke of its compression cycle. This unit comes with 14mm threads, but an 18mm adapter is available for standard aviation use (see p/n 7885). 1 lbs (0.5Kg) shipping weight.