Mopar Small-Blocks. Larry Shepard

blocks can be machined with either the standard 59-degree tappets or the 48-degree tappets. Once machined, they can’t be changed. Similar to the 50- and 60-mm cam bearings, the biggest advantage for the 48-degree tappets comes about with high cam lifts, high valvespring loads, and high engine RPM.

      Today, block castings are machined on CNC-machines, and this precision machining process centers the actual machining on the cylinder bore and centers the wall thickness. This centering process tends to reduce or cancel out the effects of any core shift. In addition, foundries today try to design the cores so that they are locked in place and can’t move once assembled. The foundry uses sonic testing to help find any broken or damaged core blocks and keep them from getting into the machining process.

Machining

      Find a machine shop that has a history of machining Mopar engines and that engine builders recommend highly. As such, a machine shop familiar with the A-engine small-block design will perform the best job. This basic process actually starts with a close inspection followed by many accurate measurements. The measuring process usually starts with a thorough cleaning of the various parts. With new parts you tend to know more about them when you bring them to the machine shop; in some cases you may have purchased them already machined. Race blocks are often shipped rough-bored so your machine shop can finish them to your specifications.

       You can make many measurements on the block, such as bore size (diameter in inches) or center-to-center distance (4.46 inches on small-blocks) and many others. The dial vernier is the quick and easy way to get rough ideas of what you are dealing with. The machine shop will use a dial bore gauge for the fine-tuning measurements. This is usually the preliminary step to boring/honing the block.

       The honing stones work on the cylinder wall, and the result is a crosshatch and roughness in the finish. The typical cross-hatch angle is about 45 degrees. The roughness you could describe as very smooth, but you generally do not want to mirror finish.

      Honing Plates

      Honing locations for A-engine and Magnum engines are reasonably common. A honing plate is a 1- to 2-inch-thick steel or cast-iron flat plate with all head bolt holes and cylinder bores. The heavy plate attaches to the cylinder block and uses the actual head bolts, which are torqued to the same specifications as the heads.

      A honing plate is designed to simulate the stresses, distortions, and wall movements that are normally caused by the installed and fully torqued cylinder head.

      It is generally recommended that the main caps be installed and properly torqued during this honing operation.

       The machine shop tends to have the honing plate and will have the special head bolts to attach it with. To do the best job, the thread engagement with the honing plate should match the thread engagement used for the cylinder head. (Photo Courtesy R. Koffel)

      Honing plates should always be used when boring and honing the cylinder block. These put the cylinder head stresses into the cylinders, so that it simulates what the cylinder walls do with the head installed. During this process the cylinder bore’s diameter is measured with a dial-bore gauge. The machine shop leaves about .001 to .002 inch of material for the final honing operation.

      Decking/Milling

      Once cleaned, the machine shop can tell how flat the deck surface is and if there is any damage (scratches, etc.) that needs to be fixed. You probably don’t need to deck a new block. The decking process tends to remove .010 to .020 inch, unless a special amount is requested. Typically, production blocks can have up to .060 inch milled off without causing problems.

       With a new block, you select the bore size based on the block’s overbore capability, but you typically leave yourself room for at least one rebuild. With a used block, once cleaned and measured, the machine shop gives you some numbers: what is takes to fix the wear that is observed, which will be something around .010 to .020 inch (and you know the maximum the block should be overbored); on today’s thinwall blocks, the overbore number is about .020 to .030 inch. Older blocks (1972 or earlier) can do .040 to .060 inch, but the old 340 blocks should still be limited to 4.08-inch max!

      Milling Calculation

      Precisely fitting all engine parts together requires extremely accurate machining. When you mill or deck any part of the engine (with the most important being the block and head), consider milling all three surfaces so everything lines up again as the engineers planned. It is too late when you find that the manifold does not fit to the head at final assembly. This is not just a concern for the block. Take the amount that you plan to machine off the block’s deck surface and add that to the amount that you plan to machine off the head’s deck surface.

      For example, if you take .030 inch off the head and .020 inch off the block, it makes a total of .050 inch. The intake manifold face cut could be made to the intake manifold, but I recommend that it be done to the intake face on the head. That way all manifolds fit, rather than having to specially machine all manifolds for this engine. If the china wall front and rear aren’t shorted the proper amount, they tend to hold the intake manifold up, and it won’t seal to the head. This is probably not required for small amounts such as .010 inch.

      You can assume that new heads are machined to fit like a production head with zero milling. New heads might be milled to change the combustion chambers to a specific size.

      Align-Boring

      Sometimes a used block has distorted or damaged main bearing caps and saddles. If you’ve measured the caps and saddles and determined that they need honing or boring, a machine shop needs to perform this service. Align-boring trues them so the crankshaft performs at its best and does not bind.

       Align-boring is required if the block’s main caps have been replaced (one or all), and upgrading to steel main caps provides additional strength. To align-bore the block, a very small amount of material is removed (milled) from the main cap’s parting line surface of each main cap. (Photo Courtesy R. Koffel)

      Stress Relieving

      Manufacturers stress relieve all new blocks; a used block is seasoned, so it has been stress relieved through repeated thermal cycles. Stress relieving removes the stresses that were introduced into the block during the casting process. Once the block has been stress relieved, it is best to take your time machining it so that you don’t introduce any more stresses into the block. Take two cuts, not just one. Take three cuts instead of two. Try boring the block in the cylinder sequence of (driver-side bank) number-3, -7, -1, -5.

      High-Performance Gasket Options

      In the past, high-horsepower A-engines used very high compression ratios (12:1 or higher), and it made cylinder sealing difficult. New O-ring designs made running these high compression ratios for racing feasible. Then the aftermarket made head gaskets with the O-ring built in (Fel-Pro, for example). Next, the aftermarket (Fel-Pro and Cometic, plus others) introduced the multi-layered-steel