Jeep 4.0 guys,check this out....

They changes the port design along with intake/exhaust manifold redesign. The earlier design was a carry over from the 4.2 which was designed in the 50's. It was a tractor engine no matter how you sliced it...the port changes were so it would meet stricter emissions requirements. They do last a long time if you dont spend much time on freeway. I bet that head costs more than one of our Magnum swap kits!!!