Originally Posted by HP_Lovecraft
Nearly all the engine components interchange between the 4.0 and 4.2.
For example, its a fairly common mod to take the head and EFI off the 4.0 and put it on the 4.2, or take the crank and pistons out of the 4.2 and put it in the 4.0.
Obviously its mostly just semantics, but the modern 4.0 was basically the same internally as the original Nash engine designed 50 years ago.
The heads will bolt up, but the oil/coolant passages don't necessarily line up completely.
AMC designed the motor to take advantage of existing molds/assembly line, but it's still a new design that shares it's lineage with the original 3.8 inline from 40-50 years ago.
Originally Posted by mustangii
Yes, almost everything will interchange, except for the crap Renault ignition and injection system, besides that would be a down grade.
Anything is an improvement over a Carter BBD carb.