My Ryobi strimmer stopped working and I have a new ignition unit to fit. On stripping down the strimmer I need to take off the outside casing round the motor to fit the new ignition unit. There is a star screw in the centre of the shaft but the motor when I try to unscrew it.
How do I stop the unit from turning so I can unscrew and release the casing from the unit and get to the ignition?

Phil Foster, October 2011