Casting flaw or over torque on the bolts are the most likely. With either of these a loose or busted hanger allowing the exhaust to flex too much at the manifold lets loose. Overheating can magnify a flaw as well. My favorite is a buddy who hosed his engine off after a long run. The cold hose water hitting the hot manifold and snapped it. When it let loose it sounded like a gun shot. :shock: However that was a Toyota so it was no big loss. :mrgreen:classicjack said:what causes an exhaust manifold to crack ?