Freezing and thawing will do it.
Overheating will do it.
I remember friends in the 70s saying "good luck finding a 216/235/261 head, they are all cracked"
They are not all cracked. But, I bet the ones in a dismantling yards have good reason to be suspect for cracks.
If I was buying a head, I'd pay to have it magnafluxed and certified to be uncracked, then I'd know I was paying for shipping a good head. I'd try to negotiate on the cost of head, due to need to magnaflux it.
