Final answer:
The defining quality of an overhead valve engine is that the camshaft is located in the cylinder head of the engine.
Step-by-step explanation:
The defining quality of an overhead valve engine is that the camshaft is located in the cylinder head of the engine. This design allows for better control over the movement of the valves, as the camshaft is closer to the valves. It also leads to improved combustion efficiency and power output. The overhead valve engine is commonly used in many modern vehicles.
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