This competition designed gasket is for a bore (4.375" to 4.540") usage on 1991 and later "Gen V" Chevrolet big Blocks with aluminum heads. It is a preflattened wire ring. The nominal compressed thickness is .039", with compressed volume of 10.5cc. These are composition design gaskets. Technical Note: This gasket is for "Gen V & Gen VI" engines only, (1) gasket per package. These gaskets contain stainless core for saltwater marine use.