Cylinder heads for 5.9L Cummins Marine 2004.5–2007 Marine engines seal the top of the cylinders. They hold valves, injectors, and air and exhaust passages. Made from strong cast iron, they handle high heat and pressure. These heads support a 24-valve design for better airflow and power. They improve combustion and engine efficiency. Built for marine use, they offer durability and long life in tough conditions.