[Amark]

The series III xj6 had a 4.2-liter engine that was designed before word war II and was used up until 1987. It was the definite week link in the late model series III. With the advent of pollution controls they were known for dropping valve guides & cracking heads. The head is held in place with something like 30-8" long studs that pass through the aluminum head & water jacket into the block. The steel studs weld themselves to the head through electrolysis making it a major chore to get the head off the engine. My Jag mechanic has a series III that he is parting out because after many hours he still cant get the head off.

The series III has a high tail gear something like 3.08 to 1. The right SBC will wake the car right up. A Tuned Port SBC with aluminum heads and a roller cam in a jag is a great conversion. A SBC weighs less than the 4.2-liter Jag motor makes more horsepower and torque and is more efficient & reliable.