Originally Posted by CJ7-Tim
Look for the head casting number near the 3 & 4 spark plugs.
1987-1990 Jeep 4.0L used head casting 2686
1991-1995 Jeep 4.0L used head casting 7120
1996-1999 Jeep 4.0L used head casting 0630
1975-1986 6-258 would get the #2685 head (if you're wondering,) and the 1999-2006 head was numbered 0331 (also, it had COP/DIS - and therefore bosses to mount the coil rail, which need not have been used - and if you see #0331 you'll want to pull the valve cover and look for "TUPY" cast into the surface on top of the head - those are the ones that are not
prone to cracking, and appeared 2001.5-2002. Ish.)
The later 258 heads still had siamesed #3 and #4 exhaust ports as well - if you only see five exhaust primaries, it's the 258 head. If you see six, it's the 242ci. I believe the 258ci manifolds were cast iron log-style jobs, and I know the 242ci manifolds were all tubular steel.)