Slant 6 Block Crack

That's a tough call. not in a water jacket so if it sealed up and did not leak oil it would probably be OK. Tough part is knowing it's there and the lingering doubts. Flywheels sometimes get what looks like cracks in them but after re-surfacing they all disappear and this might be like that if your lucky. Cause would be pure speculation and could be any one of a dozen things. It get it mag checked to see how deep it is and does it stop, as it appears to, and not extend all the way into the adjacent cavity. If it did extend in that adjacent cavity I consider a different block.