Cylinder heads on the 2004.5–2007 5.9L Cummins seal the combustion chambers and hold parts like valves, springs, and rocker arms. These years use common rail injection, so the head must handle high fuel pressure and heat. The stock cast-iron head is strong but can crack or warp with high boost, heavy towing, or tuning. Upgraded or reman heads may offer better airflow, stronger castings, and improved valve train parts. A solid cylinder head keeps compression strong and helps the engine run reliably over time.