Causes of engine block crack?

I agree w/TT, the top of that piston looks like it's peppered with detonation/pre-ignition pitting, that MUST be corrected if so.....just because You don't think You hear it doesn't mean it's not happening. I doubt it's pre-ignition because that usually causes loud popping/backfire/misfires, but detonation has killed more engines than can be counted.
Sleeve it, replace the needed parts & go.