This, in turn, may cause cracks to form as the metal cools and contracts. But elevated operating temperatures can push a head beyond its design limits causing the metal to deform. Even so, the heads are designed to handle a certain amount of normal expansion. Aluminum expands at nearly twice the rate of cast iron, which creates a mismatch in expansion rates on bimetal engines with aluminum heads and cast iron blocks. Loss of coolant, severe overheating as well as sudden changes in operating temperature from hot to cold can all create the kind of conditions that cause cracks to form. In many instances, the cracks are not the cause of the failure but a symptom of another underlying problem such as overheating, detonation or incorrect installation (wrong torque on head bolts, dirty bolt threads, etc.).Ĭracks typically form when a cylinder head undergoes too much thermal stress. Cracks are often blamed as the cause of a head failure.
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