Question 30 MODE02 - Assistant MODU Engineer

The Correct Answer is B **Explanation for Option B (Scale on cooling passages):** The cylinder head houses the combustion chamber and contains critical cooling passages designed to dissipate the immense heat generated by combustion. If mineral deposits (scale) build up within these passages, they act as an insulator, severely restricting the flow of coolant and significantly reducing the head's ability to transfer heat. This localized, excessive overheating causes uneven thermal expansion and extreme thermal stress on the metal, leading directly to the formation of cracks, often originating between the valve seats. **Why the other options are incorrect:** * **A) Overheated intake valves:** While overheating is generally detrimental, intake valves are cooled by the incoming, relatively cool air/fuel mixture and generally operate at much lower temperatures than exhaust valves. Their overheating is unlikely to be the primary cause of a massive structural failure like a cylinder head crack, though it could damage the valve or seat itself. * **C) A leaking oil control ring:** The oil control ring is located on the piston and is responsible for regulating the oil film on the cylinder wall. A faulty ring primarily causes excessive oil consumption (burning oil) and carbon buildup, but it does not directly cause the structural overheating stress required to crack the cylinder head. * **D) Heat transfer from exhaust valves:** Exhaust valves operate at the highest temperatures within the head and are a major source of heat that the head must dissipate. However, the cylinder head is specifically designed to manage and transfer this heat away. The mere presence of high heat transfer from the exhaust valve (which is normal engine operation) does not cause a crack unless the cooling system's ability to handle that normal heat (e.g., due to scale) is compromised.