Remove the side plate which covers the clutch assembly and the top cover which covers the air filter and spark plug.
Take out the spark plug as the cylinder needs to be blocked, you can buy a special plug for this or use a piece of rope.
Assuming you only have rope at your disposal, feed one end of the rope into the cylinder ensuring that the other end is free.
On older saws place a spanner on the hex nut which holds the clutch in place and turn ANTI clockwise, you will feel the piston bite on the rope just continue until the clutch is free,
re-fitting is the reverse procedure.
On some newer saws you will need to use a hammer and cold chisel / screwdriver to drive the assembly round ANTI clockwise, there should be drive points visible on the clutch cover which indicate this.
Regards!
A.L.Bradshaw, July 2006
