For a 1999 XjR will any other year heads work on the 1999 4.0 engine?
Thanks..

 

I believe the 1999 thru 2003 heads will be compatible.

 

The North American 1999MY XJR is AJ26.
Although the normally aspirated 1999 XJ8 is AJ27, the intake cams ARE DIFFERENT.
The reluctor lobes for the CMPS are different between AJ26 and AJ27.

Also, the VVT is only on the Normally Aspirated version.(the oil gallery needs to be 'plugged' if a VVT head is used)

Any 4.0 V8 cylinder head casting will work if you swap parts to make a compatible setup.
Jaguar addressed this issue with a TSB many years ago.