It is probably not a good idea to use air pressure to try to move a stuck piston. If it does move, it will move like a shot (think "spud gun"). Grease is not compressable, like air, so the piston, if it moves, will only move a fraction of an inch with each pump. I once had a 9HP Briggs that broke a connecting rod, and after we had the head off, someone apparently pushed the piston down far enough so 1 or more rings were below the bore. My BIL and I were standing next to the engine as I pressured the crankcase with 180 pound air through the crankcase breather. Nothing happened for a few seconds, and then the piston broke loose, and shot out of the bore and punctured the aluminum foil faced insulation on the 16 foot high ceiling! A second later, it dropped down past us and dented the sheet metal flywheel shroud only inches from the bore it had just shot out of! I realize this would not happen if pressurizing from the other direction (through the spark plug hole), but grease will, in any case, give a more steady movement.
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