use an old started rope with some knots in it, pull the spark plug and bring the piston past the transfer ports so the rope cannot enter into them then put the rope in the hole for the spark plug this will stop the piston from moving so you can take the clutch off (remember its left hand threads) or either get a piston stop from a stihl dealer you can get them cheap (little plastic ones) but be careful not to break it when using it.
