This aluminum Bow Tie cylinder head offers a significant performance improvement over production castings. Bow Tie heads are recommended for large displacement competition engines 1965-1996 Mark IV, Gen V and Gen VI. The Bow Tie head's intake runners are raised .100" above the standard port location and enlarged to improve airflow. The rectangular exhaust ports are raised .750" from the standard position to reduce turbulence in the runners. Vanes in the floor of the exhaust ports reduce turbulence (these are referred to as "W" ports by some Chevrolet enthusiasts). The big-block Bow Tie head has unique 115cc open combustion chambers. Material was added between the valve seats to reduce the combustion chamber volume, and the chamber walls adjacent to the valves are not relieved. The seat inserts and bronze valve guides are semi-finished for 2.19" diameter intake valves and 1.88" exhaust valves. The rocker arm stud bosses are reinforced and heli-coiled to improve reliability with racing valve springs. The rocker cover rails are raised .250"; standard valve covers can be installed. Two additional head bolt holes are drilled in valley area. Important Technical Note: The Bow Tie cylinder head's raised intake runners require valves which are .400" longer then the stock. The valve spring seats can be machined .200"; check the valve spring installed height after machining the spring pads. Pushrod guideplates (P/N 3860038) must be ground for clearance before installation. The upper intake manifold bolt holes are not drilled in Bow Tie cylinder heads. Ports in production and after market intake manifolds must be modified to math the intake runner entances. This head's new bolt holes in the exhaust manifold flange or modify the header flanges to center the exhaust pipes over the port exits the deck thickness is 9/16" for maximum strenth. The four head bolt bosses adjacent to the exhaust ports are taller than production heads; use aftermarket head bolts listed in chart. The heads are cast from 356-T6 material and the casting P/N is 14044861, same as the first design heads.