They start out with a very durable .375" billet rail to provide a superior seal to the cylinder head with steel inserts pressed in where the studs mount the covers to the head so the valve cover nut doesn't dig into the rail after a couple seasons of use.
Billet spark plug tube bungs that seal with a O-ring are part of the package as well as a simple snap ring to hold the spark plug tube in. Burn down breather mount pad is provided to fit most industry standard burn down breathers. These covers are light weighting in at only six pounds a piece.
Spark plug tubes, valve cover gaskets and mounting hardware are all sold separate but can be purchased from RBS.