Yates D3 Cylinder Heads were introduced to Nascar in 2004 and are an upgrade to the Yates C3 style cylinder head. Casting dates can be found inside the valve cover rail along with the Ford Racing Part number M-6049-D3. The valve angles were changed from the Yates C3 head as well as an enlarge Intake valve. One of the easiest ways to tell the two heads apart is that most D3 heads had a round exhaust port where the C3 was square. You cannot use a piston from an engine with a C3 head to an engine with a D3 due to the change in valve angles.