To stop the crankshaft from rotating, remove the spark plug and thread some cord* into the cylinder, rotating the crankshaft using the 5/32 allen key until the cord stops any further rotation. Using a screwdriver to snag the flywheel may cause damage to you and more importantly, the flywheel, or its housing.*Don't forget to keep one end of the cord out of the cylinder barrel!
Tony H
August 2012