Originally Posted by kcirwilliams
I've got .030" pistons. There's plenty of people that have been fine with a .040" overbore and I've even heard of people having success with an overbore as big as .060" so I'm not worried. The pistons, rings, rods, block, cam bearings, crank and head is all going to the machine shop together. They will have the pistons there so they'll be able to match the bore to them.
You'll probably be okay with .030 over but .040 & .060 over.....
Read this, it's from the Mopar Jeep Engines, 3rd Edition, manual, part number P5007161, page 134:
Boring and Milling Specifications
Because it is a thin wall casting design, the 4.0L can only
be over-bored .030" (maximum), although .020"
(maximum) is preferred. Regardless of the amount of
overbore, if the engine is to be used for high output racing
purposes, the cylinder wall thickness should be checked to
be sure the block doesn't have a bad case of core shift. The
cylinder wall thickness should be the same all the way
around or thicker on the major thrust side of the cylinder
bore; that is, passenger side (as installed in the vehicle).