Allready had this in my files.
I have an old impact chisel tool. years ago I fabricated a round flat piece of 3/8" plate and welded a big deep nut in the middle of it. When I get a stuck engine I take the head off and pour diesel fuel on top of the pistons and them take the impact with a flat ended tool in it and sit my fabricated tool on top of the piston and let it hammer. I've never failed to loosen the worst stuck and rusted engine with this thing. The crank won't move with the first two or three but when that fourth one breaks loose it will usually begin to move the piston down. Believe it or not it will usually loosen the stuck rings too because they are what is stuck to the cylinder wall and the vibration will wiggle the loose most of the time. You can tell if the rings are free by looking down beside the piston at where the end gap is and wiggling the piston back and forth to see if the rings move. Zane