Get about 10 feet of 1/4 inch rope.
Take the spark plug out, make sure the engine is at the bottom of the stroke, and now feed the rope in the plug hole until you can't fit any more. Now tighten the nut up. As the engine tries to rotate the rope will stop it. Once you are done working on it spin the engine backwards enough to remove the rope. Put the plug back in and go have fun.
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