Modern Engine Blueprinting Techniques. Mike Mavrigian
are also available for high-stress areas, such as cylinder head bolt holes. Lock-n-Stitch makes reverse-angle-pitch threaded inserts that apply greater clamping force between the insert and bolt hole walls. This type of insert can actually result in stronger threaded connections than the original tapped holes.
The only real debate regarding thread inserts involves using dissimilar metals (for example, stainless steel inert in an aluminum cylinder head, especially in spark plug holes). Some builders claim that dissimilar metals can result in different rates of expansion and contraction during operating temperatures. My own experience with threaded inserts has always been successful. Others claim to have experienced problems with fastener loosening or spark plug performance. My opinion is that stainless steel inserts in aluminum components is a good idea, since the harder stainless steel material (as opposed to the parent aluminum) greatly helps to avoid galling or thread stripping issues.
The engine block is the foundation of any build, and several critical machining processes are often required to bring the bores, deck, and dimensions of the block into precise and correct specification. When someone refers to an engine as balanced and blueprinted, this all too often means that the crankshaft has been balanced but the engine has not been blueprinted. Balancing the rotating and reciprocating components is simply one element in the process. Machining the block to achieve precise dimensions is the first and most important step.
Once the block has been cleaned and flaw checked, the first order of business is to make sure that the main-bearing housing bores are properly aligned and sized to specification. Cast engine blocks, although seemingly stout chunks of cast iron or aluminum, have a tendency to “move” as the result of core-shifting because the block “seasons” through numerous heat cycles. In order to get the block to “settle” to a more stable condition, the block can be seasoned by use (and then machined and accurized), or the process can be hastened by treating the block to cryogenic or vibratory stress relief. This allows the molecules to settle and to become more uniform, reducing the potential for future stress-related movement.
The main bearing journals and caps are honed. After a few passes, the main bore diameters are regularly checked with a precision dial bore gauge.
High-quality aftermarket performance blocks may be stress relieved at the factory, which provides a more stable block over time. Heat and stress cycling over time improves the stability of the casting. For any used OEM block, the main bore must be reconditioned, even if no apparent damage is present. It should be obvious that the main bore must have perfectly round bores of the proper diameter, but beyond this, all the bores must be aligned to eliminate any potential crankshaft bind. Over time, an aged block may have tweaked enough to create a misalignment due to heat cycling or previous engine overheating (thermal-induced warp).
When aftermarket main caps are to be used, the radius is typically smaller than needed to provide the machinist with enough material to remove to accommodate a to-size main bore diameter.
Aftermarket main thrust caps have a shallow thrust relief and must be finished-to-depth using a cutting bit on a mandrel to match the thrust area of the block’s thrust saddle.
A typical procedure for any previously used block is to align hone the main bores. The block is rigidly mounted on an align-honing machine. A long honing mandrel that is fitted with abrasive stones passes back and forth through the main bores. In order to perform align honing, the bores must be reduced in diameter to allow material removal for achieving the original bore diameter. In order to accomplish this, use a cap grinder to grind the main bearing caps at the cap-to-block mating surfaces, removing about .003 inch from the caps. All housing bore diameters must be first established to exactly the same undersize before align honing.
Align honing is performed on a dedicated align-honing machine. With the honing mandrel centered to the block main saddles, the abrasive-stone-equipped mandrel is stroked fore and aft.
This is BHJ’s LBF-1 (line boring fixture) for line boring main bearing or cam bearing bores, designed for roughing-in steel main caps or boring for installation of roller cam bearings. This allows obtaining correct cam tunnel location in all three planes, making the cam tunnel absolutely parallel to the mains at the correct center-to-center distance. (Photo Courtesy BHJ Products)
The hone self-aligns to the housing bores and corrects any main bore distortion that may have been caused by block warpage and/or main cap stretch. Centering pins on the honing mandrel allow the mandrel to be positioned at the correct crank centerline. While rocking the mandrel on the centering pins, expand the honing stones until the unit doesn’t rock. Remove the centering pins and install the main caps. Torque them to specification. Severe out-of-round condition can result if the mandrel is not properly centered.
With the honing machine’s oil lubricating and cooling the stones, the mandrel spins and is stroked back and forth. After about five strokes, remove the mandrel and check the bores with a dial bore gauge. Continue the honing process until you achieve the final diameter (bores must be measured often to avoid overhoning).
The fore/aft movement of the crankshaft, which is the thrust clearance, deserves close attention. If the thrust clearance is excessive, the crankshaft eventually pounds out the bearings and the crank’s journals. If thrust is too tight, the crank journals overheat and the bearings melt. A bad or incorrect torque converter on an automatic transmission often causes thrust bearing failure. If the converter body expands (pressure ballooning), moving by a mere .005 inch or so, it can apply excessive pressure to the rear of the crank, pushing the crank’s thrust surface against the thrust bearing.
Once the main bores have been corrected, test fit the crank with bearings. Check thrust clearance by using the following procedure. Mount a dial indicator to the block and position the plunger to contact the crank snout or flat counterweight face. Move the crank rearward as far as it goes. Adjust the dial indicator with about .050-inch preload, and zero the indicator gauge. Move the crank fully forward and note the amount of movement. Compare this to the specification. Typical thrust clearance should be in the neighborhood of .004 to .006 inch. If thrust is too tight, the crankshaft’s thrust surface may need to be reground to achieve the