These guideplates are used on 1960-90 iron heads and all aluminum cylinder heads. The big-block Chevy's unique splayed valve design uses hardened steel pushrod guideplates to align the rocker arms with the valve stem tips. This slotted guideplate is used with 3/8" diameter pushrods. 8 required. Technical Notes: Install with screw-in rocker stud P/N 3921912.