The Edelbrock #60139 Performer RPM Cylinder Heads are designed as a bolt-on performance upgrade for any 1967-91 AMX, Javelin or Jeep with a 343, 360, 390 or 401 AMC V8 engine. They include adjustable rocker studs and guide plates set-up for use with small-block Ford style adjustable 1.6:1 ratio rockers. The intake flange is dual-drilled to accommodate early and late model intake bolt patterns, covering 1967-91. Complete heads include larger than- stock valve springs to handle most high-lift cams. They feature stock location ports for compatibility with standard manifolds and headers. A direct bolt-on for 1970 and later engines with 1/2" head bolts, these heads include step-dowel pins for pre-1970 engines. Their dual-quench combustion chambers produce approximately 9.5:1 compression in 401 V8 engines. The 45° intake seat angle offers improved mid- to high-lift flow over the stock 30° angle. The Edelbrock #601139 Performer RPM Cylinder Heads have a machined heat crossover for 50-state emissions legal performance.INSTALLATION NOTE: • Match with Edelbrock head gasket sets. • Recommended spark plugs are Champion RC12YC or equivalent 14mm x 3/4" reach with flat gasket seat. • Use Edelbrock Step Washer Kit #9693 on 1967-69 engines with 7/16" head bolts. • Deck thickness: 5/8" • Rocker stud diameter: 3/8" • Pushrod diameter: 5/16" (width of slot in guide plate).