AA 500 Series Casting With seats and guides 40mm Intake 35.5mm Exhaust, It is set up to build the performance Head of your needs. 

The 500 Series Castings come STD with: Clean flowing cooling vanes, for maximum cooling through the head, and around the intake ports for increased HP and over all cooling. A modernized Combustion chamber, for increased head flow & performance. A second Full Fin, so you can Fly cut your heads deeper to achieve almost any desired Head CC volume needed. Blocked off rocker stud bosses, to eliminate the O-ring and stop possible oil leaks. Long reach 3/4 Inch Spark Plugs, to reduce the chance of cracking between the Valve seat and Plug. 

Casting: AA 500 

Valve Size: N/A

Head Bore: 85.5

Plug Size: 12mm  (Replacement NGK Number is D8EA)