World Products 043610 - Cylinder Head Cast Iron Chevy Small Block S/R 170cc 67cc 23Degree 1.940" x 1.500" Straight Plug. Bare CastingWorld Products 043610 Cylinder Head Cast Iron Chevy Small Block S R 170cc 67cc 23Degree 1. 940" x 1. 500" Straight Plug. Bare Casting Worlds S R cylinder Head are the preferred alternative to expensive OEM castings or junkyard rebuilds. These Head are designed with extra thick decks and walls for improved reliability as well as equipped with hardened steel exhaust seats that are compatible with todays unleaded gasolines. Needless to say, the S R is