The big difference is Ram Air IV's had bigger intake ports than the Ram Air II. The Ram Air II shared intake port size with the typical 1965 and up D-port head and was the first round port exhaust head. The 1971-72 455 HO and the 1973-74 455 SD had the bigger intake ports along with round exhaust ports too. The 455 SD intake ports were slightly bigger then the Ram Air IV intake ports. The 455 SD intake ports broke into the push rod holes and valve cover bolt holes. Tubes were inserted into the pushrod holes to seal the port and valve covers bolts must be in place and installed with thread sealer to prevent vacuum leaks.
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Matt Luster
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