All 4.0 inline-6 Jeep engines use pressed piston pins. The amount of interference fit between the pin and the rod is .0006 to .0021 inch. An arbor press is used to disassemble and reassemble pressed-pin rod/pistons. Heating the small end of the rod can also aid in assembly. These procedures are generally done by the machine shop. Typically, the piston manufacturer includes the pins with the piston. Aftermarket pistons are generally made so that they can be used with either pressed pins or floating pins. This means that the pin-lock grooves are machined into the outer ends of the piston pin towers. If pressed pins are used, the pin locks are not installed. The pin may be the same. Do not try to convert stock 4.0 rods to use floating pins. This would require the small end of the rods to be machined out (enlarged) to accept a bushing and this procedure would weaken the rod too much. Also, do not try to run the rod metal-to-metal (no bushing) with a few thousandths of an inch of clearance.

Find more tips like this in the book: JEEP 4.0 ENGINES: HOW TO REBUILD AND MODIFY

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