The J heads were cc'd to 67cc's and had a mild porting job. The X heads are untouched, as-removed with a whole lotta miles on them. I have not measured the X heads.
hi, usually most are about 5 to 6 cc bigger over min volumes. the X head is 63.3 cc, J head is 64.7 cc. these are set by mopar. different style valves will affect volume. nail head valve will make it smaller. it doesn't have the dish of tulip valve.