This competition designed gasket is for a bore (4.375" to 4.540") usage on 1965-90 Mark IV Chevrolet Big Blocks with aluminum heads. It is a preflattened copper wire ring with permatorque/blue stripe coating. The nominal compressed thickness is .041", with compressed volume of 10.9cc. 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.