Made from an AA 501 Series Casting, It is set up to build the performance Head of your needs. It Comes with Seats 42x37.5, Guides, with a 12mm plug hole.

The 501 Series Castings come STD with: Increased material at the intake surface giving you the ability to make much bigger ports . 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 501

Valve Size: 42x37.5

Head Bore: 85.5

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