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