4.2 is a slightly stroked 3.9 ie they have the same bore, 4.2 cam supposedly has a little more meat. The 3.5, 3.9, and 4.2 are all pretty much the same engine up until the serpentine versions came out in the 94/95 model years. They dropped a row of head bolts in the transition between 14cux and gems injection systems and switched from all metal headgaskets to composites. That switch also corresponded with going to stretch head bolts. I've put over 300k on both. Still working on getting to 300k with a bosch setup. I'd second what Jack said above, the head gaskets are a non issue (pre bosch) if the cooling system is maintained,Ie hoses are replaced before they dry rot in half and antifreeze is replaced and system flushed out ever two years. Hard to find one that has had this most basic level of care/ preventative maint.Is the 3.9 more reliable than the 4.2 or vice versa? Or are they about the same?
always surprises me how folks don't follow this.antifreeze is replaced and system flushed out ever two years
The rise of "lifetime" coolant, "lifetime" transmission fluid and ultra-high interval synthetic oil changes has had a misleading effect on maintenance philosophies. Part of the throw away society we've become.always surprises me how folks don't follow this.
even the LR dealer that serviced my RRC in Switzerland didn't think this was important.
that and changing the oil every 3,000 miles has had a very positive impact on my engine.