you guys are thinking waaay too hard. get some small diameter nylon rope, put the engine about 1/4 turn after TDC on the #1 compression stroke, and feed a bunch of it in(make sure you leave an end to pull out!). using a screwdriver to poke it down the spark plug hole helps. when you try to loosen the bolt by turning it counterclockwise, the piston will compress and lock against all the rope in the cylinder and youll be able to get the bolt to break loose.
you can also use this trick to tighten bolts, just put the crank before top dead center instead of afterward. i put my flywheel on and torqued the bolts when the engine was hanging from the hoist
