Metal fatigue?
Improper tightening procedure
Improper torque
Over heating
A plastic radiator will crack under extreme over heat conditions for example a defective thermostat or bad cylinder head gasket will cause a cooling system to over pressurise and rupture.
Can Spark Plug Removal Crack a Cylinder?No, spark plugs screw into a threaded hole in the cylinder "heads," and therefore cannot crack a cylinder [wall around the pistons].NOTE however, that if spark plugs are not removed and reinstalled PROPERLY, this process can "strip" and ruin the threads in the cylinder "HEAD."
if there is oil in the coolant system it could be a crack in the intake, a ruined intake gasket, a cracked cylinder head
Cracks can be welded and then the head can be resurfaced.
Overheating, defects from manufacture, maybe poor assembly, poor fuel and knocking perhaps.
Crack.
Moisture
crack
Crack.
There is likely a crack in the engine block.
Glass
No expansion of contents could crack internal cylinder.