Читать книгу Modern Engine Blueprinting Techniques - Mike Mavrigian - Страница 10
ОглавлениеThe 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 desired clearance.
If the block’s thrust bearing surface is damaged, it can be refaced using an align-boring bar and a special tooling bit. If the block’s thrust surface is resurfaced, a main thrust bearing with a thicker thrust face is required.
In order to bore (using cutters) or hone (using abrasive stones), mill the mating surfaces of the main caps by a few thousandths in order to create a slightly smaller, out-of-round condition (this allows material removal to achieve the final, proper bore diameter). Secure the main caps to the block, using the same main cap fasteners (bolts or studs) that will be used during final assembly. Torque the main caps to final assembly specification. This is critical.
Aftermarket performance steel main caps are generally made with a slightly undersized radius, allowing the machinist to establish a round hole of the proper diameter when the caps are fully fastened to the block. When aftermarket main caps are installed (or anytime main caps are replaced), first align bore the main bore to within about .005 inch of the desired final diameter. Then finish them by align honing to final size. When align honing, if the block and caps are cast iron, use a harder stone. For aluminum blocks fitted with steel main caps, honing requires a softer stone such as 150-grit J45 silicon carbide.
As the engine heats and thermally cycles, the head gasket must be allowed to move (slide) without grabbing/tearing. A surface finish of 60 Ra is generally okay for cast iron, but aluminum requires a smoother finish of about 12 Ra.
The block decks on a twin-bank block must not only be flat but must have the exact same deck height from the centerline of the main bore to the deck. Decks must also be parallel to the main centerline and must have the correct angle (90 degrees to the crank centerline). For blueprinting purposes, the block decks must be surfaced using specialty alignment fixtures such as those offered by BHJ, or the block must be surfaced on a programmed CNC machine.
Always keep in mind that removing material from the block deck changes deck height, which naturally affects piston deck clearance. Factoring in your crank stroke, connecting rod length, and piston compression distance, the block decks must be cut in order to accommodate the desired piston deck clearance and compression ratio. That’s why most aftermarket performance blocks usually have taller decks to achieve exactly the block deck height required for a given setup. When dealing with an OEM/pre-used block, cleanup and accurizing the decks may results in less piston-to-deck clearance than desired, in which case you may need to order pistons with a shorter compression distance.
Resurfacing is best done with milling bits to achieve the desired surface finish.
Although a surface grinder can be used, a milling operation is preferred for greater accuracy and because cutters don’t create hazardous airborne dust. Typically, resurfacing cutter inserts are made of carbide, CBN (cubic boron nitride), or PCD (poly crystalline diamond). Carbide inserts are acceptable for surfacing cast iron or aluminum. CBN inserts are very durable and accept higher cutting speeds and feeds (for faster work time), but are best suited for cast iron. PCD represents more recent technology designed for surfacing all-aluminum blocks (alloy blocks with hardened cylinder wall treatment), but can’t be used on alloy blocks that have steel or iron cylinder liners.
Inspect the decks for low spots that have not been cleaned up after deck resurfacing. The deck surfaces should be completely resurfaced, with no shadows (low spots). The head gaskets require 100-percent sealing contact.
If the bores have taper wear and/or straightness/out-of-round issues are found, the bores must be enlarged to the next available oversize. Boring involves a dedicated boring machine (horizontal or vertical) and carbide cutters. Carefully measure your piston skirt diameter. Use the piston manufacturer’s specified location on the piston. Factor in required piston-to-wall clearance. Once again, refer to the piston manufacturer’s recommendations based on type of piston and type of application. Rough boring should be done to a smaller diameter than the finished size. In general, leave about .003 to .005 inch, which is then removed during final honing.
If a very slight oversizing is needed (a .005-inch oversize, for example), you can hone rather than machine the bores. To oversize hone the bores, start with 70-grit aluminum oxide or 100-grit diamond metal-bond stones. This leaves coarse scratches on the walls, which are then removed during final honing. If using 70-grit stones to rough hone, leave about .003 to .005 inch of material. If using 100-grit diamond stones, leave about .005 to .007 inch for final honing.
During either boring, rough honing, or final honing, stop to periodically check bore diameter with a high-quality dial bore gauge. Measure at a minimum of three locations in the bore (top, middle, and bottom) and in two directions 90-degrees apart. Taper should not exceed .001 inch, and out-of-round should not exceed .0005 inch.
Honing stone type (stone hardness) can affect bore geometry, so always check with the honing machine or honing stone manufacturer for recommendations regarding stone hardness for your block application.
If you’re faced with slight out-of-round, using a softer stone can be beneficial. Thin-wall blocks may distort when using harder stones. Unsupported sections of the cylinder may tend to push out, resulting in less material removal, which results in a tight spot for the rings. Using the correct honing stones for the block material, and following the correct pressure and feed rates minimizes bore geometry problems.
Always final-hone cylinder bores by first installing deck plates to the block in order to simulate the stresses that the block sees when the heads are installed. The deck plates must be installed along with a precrushed (used) head gasket, and the fasteners must be torqued to the proper specification. Depending on the type of block and block material, as much as about .004 inch of bore distortion can occur when the cylinder heads are mounted and the head fasteners are fully torqued. Deck plates mimic this clamping load and bore distortion, so when the heads are installed, the bore geometry is established in a more uniform manner.
Inspect the condition of the cylinder wall surfaces. If scratches or scoring are evident and more than .025 inch deep, overboring is necessary. If there isn’t enough wall thickness for moving to the next size overbore, the likely option involves sleeving or replacing the block. During an overbore, the cylinders are bored to a diameter that is slightly less (or tighter) than the desired final diameter. This leaves enough material for honing, during which the final diameter or surface finish is achieved. An overboring operation typically results in an undersize of about .005 to .007 inch, leaving this amount to be removed during the honing process.
The main caps must be installed and fully tightened to spec, and then the block can be placed in the honing machine. This simulates the stresses introduced into the block, which affect cylinder bore geometry. Torque the main cap fasteners to specification and follow the torquing sequence used for final assembly. Also it’s best to use the same fasteners that will be used during final assembly.
If you plan to use main cap studs, install them now, prior to honing. When installing studs, do not overtighten them into the block. The clamping load of caps to block is achieved when tightening the nuts. The studs should be installed finger tight, with an added nudge of no more than 10 ft-lbs. In every case, read the stud manufacturer’s instructions regarding installation and any required preload. If the main caps have side bolts in addition to primary cap fasteners, be sure to install them as well, again following the recommended torque and sequence specs.
Always use a deck plate or torque plate for the honing process. It bolts to the block deck and is torqued to the same spec required for the cylinder head mounting. This plate simulates the installed cylinder head and places similar stresses inside the block, which affect cylinder bore shape. If you hone the cylinders without a deck plate, you may achieve a nice round hole, but when the heads are bolted on, some cylinder walls can distort into an out-of-round (or barrel) shape. By using a deck plate, you’re making an effort to establish consistent round holes in the assembled state. When bolting a deck plate to a block, use a crushed head gasket of the same type that you plan to use during assembly. Remember that you’re trying to simulate the condition the block faces when fully assembled.
Honing-stone pressure and feed rate is regularly monitored during hone-stroking steps. In between passes, a dial bore gauge is used to monitor current diameter and to check for out-of-round and taper. In order to maximize honing accuracy, cylinders are honed in a staggered sequence because of the heat generated by honing. For instance, after honing number-1 cylinder, honing commences at a cylinder location away from the first bore (the order might be number-1 cylinder, -3, -2, and -4). This gives each honed bore a chance to cool before honing the adjacent cylinder location.
Cylinder overboring is performed with high-speed cutters. Again, for purposes of block blueprinting, the best method is to take advantage of CNC programming to create bores that are precisely on-center per design specs.
Cylinder honing must always use deck plates. Deck plates simulate the installed cylinder heads, placing stresses in the block that it sees when assembled. This greatly improves cylinder wall geometry because the stresses imposed by clamping the cylinder head fastener slightly distort the bores slightly. When installing deck plates, precrushed head gaskets must be in place, and all cylinder head fasteners must be torqued to the same value (and in the same sequence) as the block experiences during final assembly.
Initial honing creates a microscopic series of peaks and valleys. The peaks, the high edges of the honing scratches, present a wear-in/break-in challenge for the rings.
Depending on the design of the block, also consider other highly-stressed, component-mounting locations that can affect the shape of the cylinder bores. Here’s a good example: A few seasons ago, my endurance race team ran a pair of Dodge Neons with the 2.0L Chrysler engines (4-bangers). As usual, we first honed the cylinders using a deck plate. After running the engines in track test sessions, we tore down the engines for inspection. We noticed that number-1 and -2 cylinder bores had retained their shape Number-3 was slightly out of round by about .0006 inch. Number-4 cylinder bore had been pulled out of round by about .015 inch, especially near the upper half of the bore. The reason was that the transmission bolted to the rear of the block with the upper two bolt locations very close to the rear cylinder.
After discovering this, subsequent blocks were honed with not only a deck plate, but with the addition of a thick plate bolted to the rear to simulate the installed transmission. After running a 24-hour race, teardown showed minimal out-of-round at number-3 cylinder at about .0001 inch and number-4 to the tune of about .0003 inch—a huge improvement.
You may think that an engine block is a beefy, solid chunk of material, regardless of the block (OEM, aftermarket, cast iron, cast aluminum, or billet aluminum). However, while cylinder bores may be machined perfectly round from top to bottom, cylinder shape becomes “alive” under mechanical stress and thermal changes. Bores do change their shape during engine operation, however minimal. In the attempt to obtain maximum ring performance, it’s important to understand this phenomenon and to do our best to address this and minimize these changes. (Photo Courtesy MAHLE Clevite)
Certainly, stressing the rear of the block during honing isn’t needed for every engine block, but it’s food for thought. Study the block and try to evaluate the need for stress simulation in various areas that could potentially affect cylinder bore geometry.
If you want to go a step further, consider mounting the engine mounts and the water pump as well, simulating all the mechanical stresses that a block experiences. Admittedly, for a street application, aside from a deck plate, stress simulating with other added parts is just going to be a waste of time. However, even for a race engine, where you’re trying to squeeze every bit of power and durability out of that engine as possible, going to such lengths probably doesn’t provide a real-world benefit, but in theory, it can’t hurt.
Most aluminum blocks have iron cylinder sleeves. OEM production blocks (a GM LS2 block is seen here) are notorious for featuring a rather sloppy tolerance range for cylinder-bore centerline locations. These bores were honed to size; notice the difference in liner thickness from one side to the other. This is due to small amounts of movement in the casting core. Because these liners are fairly thin to begin with, and due to the slight offset of the existing centerline, OEM blocks with integrally cast-in liners can only be slightly overbored or overhoned (usually about .005 to .010 inch oversize is all you can hope for).
Before honing, take a close look at the bottom of the cylinder bores. Depending on the specific block, it may be necessary to use a hand grinder to knock a bit of material from the webbing surfaces below the bores (for example, the center three cylinder areas where the bores Siamese and have small humps in the casting), to prevent damage to the honing stones. This takes only about 10 seconds per spot, so it’s no big deal.
Block accurizing on a CNC machine (providing the program is written for the block at hand) is relatively easy and requires no additional guide/indexing fixtures. Here a digital probe obtains existing cylinder-bore locations. With the theoretical centerline already programmed, the boring operation takes only a few minutes for each bank.
With the block indexed at the crank centerline, an overhead cutter makes short work of deck surfacing that is parallel to the main bore and with each bank’s deck equidistant from the main-bore centerline.
Depending on the block material, the machinist must select the appropriate grade of honing stone. For a high-nickel-content block (Dart iron blocks, for example), you may need to begin with 500-grit diamond stones with a high load setting to hog out the bores to an initial diameter, followed by a final honing pass to remove the remaining material. Following honing-to-size, all cylinders should then be treated to four passes with silicon carbide brushes at 30-percent load for a plateau finish. A plateau finish essentially “evens out” the minute peaks and valleys created by the honing process. This provides a more uniform micro-finish and aids in faster ring seating as well as superior oil retention.
Several CNC equipment manufacturers provide already-written programs for popular blocks, including not only OEM blocks, but also several popular aftermarket performance blocks. An experienced machinist is also able to write a specific program for a given block, and is able to adjust machining operations from the prewritten programs.
Before honing, check the bottom of each bore. If a ledge or hump exists at the bottom of the bore (casting material of the main web area), the machinist should remove the obstruction with a hand grinder to prevent the honing stones from hitting this at bottom dead center.
Each cylinder is constantly measured for roundness prior to and during the honing process. Diameter checks are measured near the top of the cylinder, at the middle height area, and at the bottom.
Cylinder honing must be performed on a dedicated honing machine (manufacturers include Sunnen, Peterson, Kwik-Way, and others.), where stone diameter adjustment, stone load pressures, and dwell time are easily adjustable and monitored. Do not attempt to hand-hone your cylinders. This isn’t a backyard operation.
Never assume anything. Whether you plan to use OEM or aftermarket pistons, always measure each piston skirt diameter at the skirt height location specified by the piston manufacturer, for your final cylinder diameter, including desired piston-to-wall clearance. Final cylinder diameter is always based on the actual piston-skirt diameter.
Honing coolant is constantly supplied as the honing stones rotate and travel through the cylinder, keeping the stones clean and reducing heat buildup.
Plateau finishing (or plateau honing) is a popular final step following finish-honing-to-size. Using dedicated plateau brushes on the honing machine shaves the tiny peaks left by honing scratches, to provide a better surface for the rings. Essentially, this honing step “breaks in” the cylinder wall finish (evens out the peaks and valleys), which provides a more uniform surface finish while maintaining proper ring bearing area for oil control and ring lubrication. This process also extends ring life (since the rings aren’t forced to wear off these peaks). Today’s piston rings are lapped at the factory for quicker break-in/seating, and don’t require a rough cylinder-wall finish for break-in. Plateau honing immediately follows final honing-to-size and only requires a few short passes using 150- or 220-grit stones. This is followed by plateau finishing.
The plateau brushes finalize the cylinder wall finish by averaging-out the miniscule peaks and valleys left by the honing stones. This provides faster and more consistent piston ring seating/sealing (the rings break-in quicker) and aids in cylinder wall oil retention.
An engine block’s cylinder bores do not remain round and true during engine operation. Even though the cylinders may be machined perfectly round, during engine operation (and especially in the case of severe-duty operation such as racing), the cylinder profile can easily change due to heat and cylinder pressures. These changes also result from molecular changes in the block material as it ages or seasons due to thermal expansion and contraction. This is referred to as cylinder bore distortion. This is a naturally occurring phenomenon.
The goal of an engine blueprint job is to recognize this and attempt to minimize bore distortion. Cylinder bore distortion also results from engine assembly. As the cylinder heads are installed, stresses are placed on the block as the cylinder head fasteners are tightened. The pulling force that results from tightening the head bolts (or tightening the nuts on head studs) can cause slight shifts in the cylinder walls, which can lead to less-than-ideal piston ring seating. Much of this is dependent on the block material, placement of the cylinder head bolt holes, cylinder wall thickness, etc. Some blocks are more susceptible to geometric shifts in the cylinder bores than others. In addition to the stresses imposed by the cylinder head bolts, tightening bellhousing bolts and other attached components can affect the geometric shape of the block.
A good example of bellhousing-induced cylinder bore distortion relates to the four-cylinder Chrysler Neon engine block. When my team ran a pair of Neons in 24-hour races, we found during post-race teardown that the rear cylinders (number-3 and -4) showed clear evidence of bore distortion, with unevenly worn piston rings and obvious pressure points in the cylinder walls. Even though the blocks had been honed with a deck plate torqued to the block to simulate the cylinder head, the stress imposed by the bellhousing bolts had pulled the two rear-most cylinders slightly out of round. Once we were aware of the problem, subsequent blocks were honed with a deck plate and a bellhousing plate, both torqued to assembly specification values. By simulating the final assembled stresses at the head deck and the rear of the block, our piston ring seating remained much more consistent, and blow-by and oil loss was nearly eliminated.
Three views of a typical dynamic bore distortion scenario. (Photo Courtesy MAHLE Clevite)
Even though you may think that an engine block is a massive and strong component, it does experience movement that can and does affect the geometry of the cylinder bores. Regardless of the type of engine, it’s vital that you simulate, to the best of your ability, the assembled stress that the block experiences before the cylinders are final-honed to size.
Engine development researchers often take advantage of sophisticated laboratory equipment that allows them to “map” a cylinder bore in order to obtain a clear dimensional picture of how that bore is shaped, from top to bottom. APAT gauge system (inner contour meter) travels vertically through the cylinder bore centerline. It has a sensitive probe that monitors the cylinder bore, which provides a dimensional view of the entire bore. This provides an overhead radial view and an isometric view at various angles
Since most engine shops don’t have access to this level of equipment, bore diameter checks are made with a calibrated bore gauge before, during, and after cylinder honing. Bore diameter measurements are taken from top to bottom of the bore in four height locations, and at four clock positions (12, 6, 3, and 9 o’clock as viewed from overhead). If you measure a cylinder with the block in a relaxed state (no deck plate), and then take measurements at the same locations with a deck plate installed and fully torqued, it is very common to find different readings. This is clear evidence that the stresses caused by the cylinder head can affect cylinder bore geometry. Once you realize this, it should become clear that you should always hone cylinder bores with a deck plate installed.
An additional factor that can affect cylinder bore shape relates to the cylinder head. Depending on the hardness level of the cylinder head material, different levels of stress can be placed on the block, although they are beyond your control. This becomes important when swapping heads (for example, changing from cast iron to aluminum) because it could result in a change, however slight, to cylinder bore geometry.
Cylinder bore shape can and does change when bolts are tightened and during engine operation. You can minimize the effect of cylinder bore shape changes by prestressing the block during cylinder honing. In addition, always follow the same routine with regard to component installation. For instance, the same torque level and the same tightening sequence/pattern should be followed every time a cylinder head is installed. This includes the installation of the deck plate for honing, and every time the cylinder head is installed to the block. Maintaining this consistency of head bolt tightening helps to minimize changes in stresses to the block.
A deck plate should be installed and torqued to the required spec for both cylinder banks before a hone is performed. On a V engine, that means installing a deck plate on both banks. From a practical standpoint, however, most engine shops may only have one deck plate for a given type of engine. Using the same deck plate for both banks is common practice and certainly is acceptable. But if two identical deck plates are available, it’s a good idea to install both at the same time.
Also, always install a head gasket along with the deck plate. This further simulates the stress that the block sees when assembled. Be sure to use the same type of head gasket that will be used during final assembly, whether composite or multi-layer steel (MLS). Preferably, use a head gasket that has already been crushed (a used one). It doesn’t “settle” as much as a new gasket.
The type of head gasket is a consideration. MLS gaskets offer greater consistency of clamping load than a composite gasket with a wire ring “crush zone” around each cylinder bore. Although an MLS gasket typically relaxes by fewer than 10 percent after initial installation, a wire-ring type of gasket can relax by as much as 15 percent or so. In other words, MLS gaskets provide greater consistency in maintaining the clamping load between the head and block.
Two types of MLS gaskets are available: active and stopper. An active MLS cylinder-head gasket has a series of metal layers with gaps between the layers and an elastomer layer that allows the gasket to compress and seal with a slight spring effect. This helps to reduce block and head deck distortion. Stopper MLS gaskets include a “dead-stop” layer and rigid cylinder bore seals, which provide additional sealing under high cylinder pressure conditions. However, the stopper MLS gasket places more distortional stress against the cylinder head. When deciding between the two styles, it’s best to follow the recommendation of the gasket manufacturer for your specific application.
I’ve already discussed the importance of prestressing the engine block prior to honing the cylinders. The importance of mechanically stressing the block with a deck plate is paramount. Always hone the cylinders with a deck plate installed. With that said, you can also consider the stress imposed as a result of heat. Although the block experiences stress that affects cylinder bore shape when assembled (primarily as a result of cylinder head installation), cylinder bore geometry is also affected by engine operating heat.
In the pursuit of optimal shaped cylinder bores during operation, some builders use a process called “hot honing.” In simple terms, the block is connected to a heater that circulates hot water at a predetermined temperature. Cylinder bores are then honed with a deck plate and with hot water that elevates the block to a temperature that approximates operating water temperature. In theory, this more closely mimics the stress and temperature that the block experiences when it runs. The practice of hot honing is embraced by some engine builders and viewed as unnecessary by others.
As engine temperature increases, a cylinder wall commonly tends to assume a slight barrel shape as a result of thermal expansion. This expanded area has inconsistent high and low spots. The result is a cylinder that is slightly out of round and no longer uniform. However, the engine’s pressurized cooling system tends to partially counteract this as the water jackets that surround the cylinder try to push against the cylinder. If the builder plans to use a hot-honing process, the coolant must not only flow through the block, it must also be pressurized.
Damaged lifter bores can be over-bored, fitted with bronze liners, and then honed for proper lifter oil clearance. In addition, lifter bores can and should be corrected to attain the proper centerline and angle by using specialty indexing fixtures or by CNC machining. OEM blocks typically require these procedures because sloppy factory machining tolerances are common in mass production. But high-quality aftermarket performance blocks already have correct geometry, and may require only final-honing for oil clearance.
If you’re dealing with an old OEM block, pay attention to each lifter bore. Just because the lifter bores don’t look scored doesn’t mean that they’re all the same size. When an OEM block is machined to correct lifter bores, all bores are likely machined at the same time using multiple tooling. On occasion, one or more lifter bores may have been machined slightly oversize (to correct tooling flaws), in which case the factory may have installed oversize lifters in certain locations.
A CNC cutter spot faces lifter bore roofs in preparation of lifter bore centerline correction. With a programmed CNC machine, no indexing fixtures are needed for lifter bore correction.
Measure each lifter bore for diameter. If you have oddball sizing, your best solution is to have the lifter bores accurized. The lifter bores should be oversized using the theoretical (correct) bore centerline to correct any centerline off-center and/or lifter bore off-angles. The lifter bores must be in the correct plane to the camshaft. OEM lifter bores are usually held to within .010 inch or so of plane, but this can be corrected by realigning, using specialty fixtures or on a CNC machine. The bores are then fitted with bronze bushings and honed to size to achieve the oil clearance needed for the lifters.
Also pay attention to the lifter bore finish. If the block was shot peened, the lifter bores may have been peened-over. The resulting “dimples” reduce oil clearance. Honing the bores with a rigid hone allows you to control material removal. A brush-type hone skips over the dimples and doesn’t solve the issue.
As with the main bore, the crankshaft bores must be accurately aligned to prevent camshaft bind and isolated camshaft bearing wear. Another aspect relates to crank-to-cam bore centerline distance and parallelism. Quality aftermarket performance blocks are CNC machined to accurately locate both centerlines, but OEM blocks may often have centerlines that are a bit off, or have a slight non-parallelism between the main bore tunnel and the cam bore tunnel. This aspect relates to the blueprinting/accurizing concern.
Lifter bore correction and/or reconditioning involves overboring the lifter bores using a precision reamer, followed by installing bronze sleeves that are then honed to size. For blueprinting purposes, the overboring is best accomplished by using a specialty lifter-bore accurizing fixture (such as one from BHJ) or by CNC machining. Either approach allows the machinist to correctly place lifter bores at the designed on-center axis and correct for any factory deviations in lifter bore angle.
Before installing bronze lifter bore inserts, oil feed holes must be drilled into the inserts (inserts are also available predrilled). Depending on the type of block involved, it is common to use a smaller oil feed hole to improve oil delivery to the main bearings. The example seen here is a Pontiac 455, where the excessively large lifter bore oil feed holes are reduced to about .040 inch.
If a cam tunnel centerline is incorrect, or if the cam bore locations are not aligned, it can be corrected by centerline align-honing the cam tunnel in relation to the main bore. Since (in an overhead-valve engine design) there are no camshaft caps to shorten in order to recreate the original bore diameter, the cam bores can be bored or honed oversize, requiring oversize-OD cam bearings (thicker shells to accommodate the original cam-journal diameter).
