To keep from having to remove the back to stop the engine from rotating, an old spark plug with 14mm threads (same as stock plug) and about an inch long can be used. Remove the electrode an anode from the bottom of plug to prevent damage to the piston. Remove your original spark plug and screw your new "special tool" into head.
This should contact the piston near the top of stroke .
Just remember to replace the "special tool" with your regular spark plug after you have reassembled the clutch.
Jim Hughes
April 2009