Since the valves are in the block beside the pistons, the pesky ones go from a cylinder bore over into a valve pocket - either exhaust or intake. These happen from either overheating OR they happened years ago as the core was settling from internal stresses. These can usually be pinned and machined over, then the cylinder is sleeved and the engine is good to go.
Fatal cracks are usually caused by freezing. They show up along the pan rail inside the crankcase or in one of the main bearing webs. At the current time these cracks reduce a block to scrap metal.
Another area is the head bolt hole-to-water jacket area. Cracks here can just be ignored - some guys joke that Ford even gave those cracks a part number..........
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