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