Jeep Engines - Oljeep

132 MOPAR PERFORMANCE PARTSStress-RelievingNote: The following is not required for thermally-cycled,used engine blocks, except for align boring and honing,which may be required on any new or used engine.The first step in obtaining straight and round bores is toselect a stress-relieved block. Only 426 Hemi engine blockswere stress-relieved in production. However, a block willstress-relieve itself with use; that is, a used block hasalready been stress-relieved (thermal cycles are the key).Heating up and cooling down is the ideal cycle - again andagain and again, which means that an engine block used inan around-town commuter vehicle is better than a blockused in a long-haul truck or police car, which isinfrequently shut off.Blocks will also stress-relieve themselves in racingapplications. A block can be built into a race engine and runseveral hundred times. It can then be rebuilt into a betterengine because the bores will be straighter and will move lesswith use. The straighter the cylinder bore, the more power theengine will make. Once you’ve got a good one, keep it.Once the engine is disassembled, the first step in preparingthe block is to remove the main bearings caps, clean alljoining faces, reassemble and torque to specifications. Thestress-relieving process consists of heating the block andmain caps to 1050°F and holding at that temperature fortwo hours. It should then be furnace cooled to 500°F at acooling rate not to exceed 200”hr. The block can then beair cooled to normal air temperature.After the block has been stress-relieved by the ‘oven’method (not by thermal cycling), new camshaft bearingsand core plugs should be installed. The old main cap boltsshould be thrown away. The block should be re-honed andthe deck flatness checked and a light cut taken if it is notflat. The main bearing bores will also have to be checkedand align-bored or honed if out of alignment. New camshaftbearings will also be required.Sonic TestingIf the block is to be used in serious racing applications, itshould be “Magna-checked” (sonic tested). With thisprocess, the thickness of the cylinder bore walls can bechecked nondestructively. This will indicate how good yourparticular block is, or it will allow you to select the bestblock of several available, if you have the option. It ishighly recommended that you have more than one blocktested so you don’t invest your time in a weak or “flexiblock.”If you only have one block, the bore-wall thickness checkreally only serves as base information and a generalperformance indicator. Write this measurement down andkeep it in your engine build-up file.However, if you have several blocks to choose from, thebest engine will be the block with the least amount of coreshift (bores with the same thickness all the way around), orthe block with the core shift in the “major thrust” direction.The major thrust direction is to the passenger side of thecylinder bore as the block is installed in the vehicle.Core ShiftIt seems as though every racer you talk to uses the term“core shift.” It’s used so frequently it seems to be the causeof every engine problem in racing! Obviously, this isn’tcorrect. Without making it too complicated, let’s look intothis subject a little closer.First of all, to discuss core shift we must have a casting. Itcan be of any material (aluminum, iron), but it has to be acasting. Second, the casting must have a “core” in it. In anover-simplified explanation, a “core” is used to cast aninternal passage in a casting. This internal passage could bea water jacket in a head or block, or an intake runner in amanifold. Not all castings use cores. Core shift is mostoften discussed relative to cylinder blocks, so we’llconcentrate on them. On any given part, there may be morethan one core. This varies from one manufacturer toanother. Cores are generally made of sand. The sand is heldtogether with a bonding agent.The cylinder block is the largest casting that we make. Themain core in the block is the water jacket. In this case, coreshift would occur if the water jacket core moved relative tothe main tooling. Since the water jacket core is set insidethe main tooling, it is possible for this core to be shifted inposition. In most cases, this shift is caused when the moltenmetal hits the core and moves or offsetsldeflects it.The core could move up or down, fore or aft, but theimportant direction is left or right. Left or right shifts movethe core in the major or minor thrust direction. This makesthe cylinder bore either thick or thin on the major thrustside. Having the core shifted so that the major thrust side issomewhat thicker is superior to being perfectly round.Consequently, having the major thrust side too thin is NOTdesired. To find out if this has occurred, the block must besonic tested.Most of the confusion relating to core shift occurs becauseracers try to find a “quick” way to determine if a block hascore shift without sonic testing! Remember that not all coreshift is bad. Looking at the front or rear of the block will nottell you if the block has core shift because you’re notlooking at the water jacket. The only way to see the waterjacket is too look down the holes in the top of the block.These holes are quite small and don’t allow you to visuallyjudge for core shift.So that brings us back at sonic testing. Sonic testingmeasures the thickness of the cylinder bore in the major andminor thrust directions. Technically speakmg, all the boresshould have the material shifted in the same direction..“