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Understanding static and dynamic cylinder bore shape.
by Mike Mavrigian
All illustrations courtesy Mahle-Clevi
This stress graph illustrates how a cylinder bore may be distorted from round to out-of-round as a result of head fastener loads.
This dimensional “isometric” view further illustrates static dimensional shift that can occur from cylinder head installation.
Even with the best of intentions and preparation, some degree of cylinder bore distortion is likely to occur under dynamic stress (heat and pressure). The challenge is to understand how these changes take place and to establish procedures that will minimize these changes. As the block ages and/or is exposed to thermal changes, the casting’s molecular structure will change, however slightly, which will affect bore geometry. In addition, distinct changes in bore shape will take place as the block is assembled. The clamping forces that result from the installation of cylinder heads will, in most cases, cause the bores to “morph” as the fasteners pull and squeeze metal adjacent to the bores.
Sometimes these changes are insignificant, while in other situations, the change can be so dramatic as to cause measurable ring dragging and subsequent loss of power, fuel economy and sealing due to both out-of-roundness and frictional heat. In other engines, these problems may be compounded if the specific block is severely affected by additional distortion that results from clamping forces caused by bellhousing bolts, water pump bolts, motor mount bolts, etc.
Granted, only the more severe cases will be a cause of concern on a street engine. However, if the goal is to produce an extremely efficient racing engine, you must consider every factor that can affect dynamic ring shape.
As noted by Sunnen’s Tim Meara, in order to accurately “map” a cylinder bore to obtain a clear dimensional picture of how that bore is shaped from top to bottom, a special “PAT” gauge is used. This is an inclinometer that features a shaft and probe. The shaft is affixed to the deck or torque plate and runs vertically through the bore centerline. The probe runs along the shaft vertically and monitors the bore walls radially. This provides a dimensional perspective view of the entire bore relative to the theoretical bore centerline.
(Don’t expect to run out and buy one of these, as they cost around $175,000 and are primarily used by piston ring and honing equipment manufacturers for analysis applications.) This allows plotting the actual shape of the bore in addition to bore diameter. The readings can be displayed radially (viewing the bore from overhead) to show where the bore shifts from the centerline and in an isometric view (side-angle perspective in a variety of view angles) that allows you to see the entire bore in a dimensional manner.
In order to obtain bore diameter readings in your shop without the use of this sophisticated equipment, the cylinder walls can be measured with a bore gauge at four different levels at four clock positions (12, 3, 6 and 9 o’clock). Once these numbers have been recorded, place a honing plate (one per deck) on the block, torque the plate(s), and re-measure the same points to reveal differences that have occurred. However, bear in mind that this will not reveal concentric bore diameter shifts relative to the centerline.
Another variable relating to bore distortion is the cylinder head itself. After measuring the bores (accessing from the bottom of the bores) on a relaxed block (no heads or torque plate), install and torque a cylinder head, and read the bores again from the bottom of the bore to note the changes that have taken place. Next, remove that head and install a different head of the same type and measure the bores once again. If you find a different distortion level/pattern, don’t be surprised. Depending on the makeup of the cylinder head, the clamping forces may reveal a different situation due to the structure of the head, especially on cast heads, due to differences in the hard/soft internal makeup of the casting core. If the head pulls down harder or softer in various areas, this will accordingly affect how the block is stressed, resulting in variations of cylinder bore shape.
As mentioned earlier, when measuring a bore for shape, we need to remember that, depending on the instrumentation being used, we may or may not obtain a true bore shape. If we use a bore gauge, we are simply measuring bore diameter (and out-of-round) in specific height levels of the bore, but since we are not referencing from the true bore centerline, we may be overlooking shifts of the centerline at various height locations. If in doubt, or if we know that the radius of the bore has shifted relative to the centerline, we can use a precision bore-truing fixture to correct the problem (such as those offered by BHJ, CWT, etc.), or take advantage of CNC technology in order to create a “new” bore centerline based on blueprint data from the block manufacturer. The bores can then be “clean-bored” to an oversize, with the cutters referenced from a fixed (and presumably correct) centerline. This will create bore trueness in a static condition, but subsequent changes may still occur when the block is subjected to loads, pressures and temperature.
Keep in mind that when we’re dealing with cast-iron blocks, the very nature of the casting process and the material mix can and will create differences from block to block. In other words, if you lined up five blocks with identical casting numbers and identical age, you’ll probably find five different variations of bore distortion. Knowing this, we need to temper our view of block analysis. Just because one small-block Chevy of a particular series and casting shows unruly bore distortion in number 3 bore, this does not mean that this condition will be exactly repeated in every block of that vintage and type. Each block casting is its own animal and needs to be treated as such.
Mahle-Clevite’s Bill McKnight was kind enough to share bore distortion information that was part of a recent Dana/Victor/Clevite NASCAR engine builders’ seminar presented by Victor Reinz engineer Ernest Oxenkonect, that addressed this very topic. Selected excerpts from that seminar follow.
Primary causes of bore distortion in relation to the cylinder head/block joint include clamping load, cylinder head gasket/combustion seal design, the honing procedure and assembly procedures.
Boring a true (round) and centered hole per blueprint specifications is achieved by either using specialty fixtures or via a CNC machine that creates a true bore centerline.
Even with the use of a deck plate, although a statically-round bore may be achieved, cylinder wall profile may change when subjected to operational heat and pressure.
Three views of a “typical” dynamic bore distortion scenario.
This graph illustrates how cylinder concentricity can be affected by head clamping, operation with regard to temperature and operation with regard to wall temperature and gas pressure.
Tags: CNC, CYLINDER BORE DISTORTION, CYLINDER BORING, CYLINDER HONING, ISOMETRIC VIEWS
