Jeep Engines - Oljeep

4.0L POWER TECH IN-LINE 6 (BLOCK) 131Block HardnessIFIRING1 5 3 6 2 4awlsROTATIONFigure 4 - 2Oversize and Undersize Component CodesJ908D-7Some engines may be built with oversize or undersizecomponents such as:Oversize cylinder boresOversize camshaft bearing boresUndersize crankshaft main bearing journalsUndersize connecting rod journalsThese engines are identified by a letter code (Figure 4-3)stamped on a boss between the ignition coil and the distributor.IWith any cast iron block, there is a relationship betweenhardness and strength. The harder it is, the stronger it is. Theassumption is that a stronger block can make more power.However, if the basic design of the block is weak, then itwill break frequently. Standard production blocks are notdesigned as strongly as “race” blocks. So if you double ortriple the power output of a standard production engine withhigh performance parts, you will have problems.We know that hard blocks don’t break and soft ones do.Racers therefore want hard blocks. If you add a strongdesign to a hard material, you will have a race block. Thismeans that the block will last longer and make more powerfor longer periods of time. Chrysler-Plymouth-Dodge-Jeephas always used a high nickel material in its cast ironblocks. This makes them harder.Rough Bored BlocksRough bored blocks have the cylinder bores roughed-in butnot finished-the surface finish on the bore is rough butmachined. That means that each cylinder must bebored-to-size and then honed. The bore-to-size operation isreferred to as the finish bore. As an engine builder, youmust leave enough material on the finish bore operation toallow for the finish hone to size the bore correctly.The advantage of rough bored blocks is you have a greaterselection of bore sizes available to you. Most race enginebuilders DO NOT want a production hone, so why payextra for it if you are going to change the bore size? Byomitting the finish bore and honing operations, the blockcan be purchased for less money.CYLINDER BLOCK PREPARATIONThe first step in preparing the block for all-out racing is tostress-relieve it (typically not required for new race blocksor many used blocks). For more details, refer to Stress-Relieving, in this section.IFigure 4 - 3IAfter the block has been stress-relieved, new camshaftbearings and core plugs should be installed. The old main capbolts should be thrown away. The block should be re-honedand the deck flatness checked. A light cut should be taken ifit is not perfectly flat. The main bearing bores will also haveto be checked and align-bored or align-honed if out ofalignment. New camshaft bearings will also be required.Block-to-Head Distortion(Refer to Head Distortion, ‘Cylinder Head’ section ofthis chapter.)