[John Brown]

56 and early 57 had 9 fins. From then on it was 7 fins. Staggered bolt pattern ended sometime in early 1959 when Chevrolet added the cooling passages in the spark plug area of the heads and changed to the straight across pattern. There was also a change in length of one head bolt per head at the same time. So, covers will bolt to 1955 heads (although 55's never came with cast aluminum covers) and up to and including early 1959 heads.