The Edelbrock #50159 Performer Cylinder Head is designed as a bolt-on performance upgrade for 1987-2006 Jeep’s with a 4.0L inline six engine. They feature better flowing intake/exhaust ports along with a modern combustion chamber design and backcut and swirl polished valves for improved flow. Retains the stock valve angle, pushrod length and valve cover bolt pattern for use with factory components and off the shelf accessories. Larger reinforced rocker bosses for improved strength at high RPM and Heli-Coil® threaded inserts in the intake manifold bolt holes for added strength as well as a 3/4" deck throughout the head also promotes stiffness and durability. This cylinder head is machined to accept both distributor or coil pack ignition. To optimize airflow, this cylinder head is designed with a 1991-2006 4.0L intake port location and 1991-98 4.0L exhaust port size. As a result, this cylinder head must be used with a 1991-2006 intake manifold and a 1991-1998 exhaust manifold. Aftermarket exhaust headers can also be used in place of 1991-1998 exhaust manifolds and MUST be used in place of 1999-2006 exhaust manifolds. Most aftermarket exhaust headers are compatible, but please check with the exhaust header manufacture to verify. This head is NOT compatible with 1999-2006 OEM exhaust manifolds.INSTALLATION NOTE: • This cylinder head is 50-state emission legal for 4.0L engines when used with factory Fuel Injection calibration and components (E.O. Number D-215-94). Pending 50-state emission legal for 4.2L engines when used with a Mopar Fuel Injection Conversion Kit (E.O. Number D-265-21). • Recommended spark plugs: Champion RC12YC or equivalent 14mm x 3/4" reach with flat gasket seat. • Recommended gasket for 4.0L: Edelbrock #7348 or Fel-Pro #26211PT. Deck thickness: 3/4".