Use this tool with a degree disc, so that disc can be stopped a certain distance before top dead centre, degree disc is marked, then crankshaft is turn round opposite way until crank stops again, and degree disc is marked, the point in between each mark is the true accurate top dead centre , note that pistons do not move for approximately 5 degree of crankshaft movement when at top dead centre, so piston at top dead centre is not accurate enough to to ignition timing.
