Cylinder Bore Distortion a Look at Static and Dynamic Cylinder Bore Shape Issues
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A CNC machine can map the desired bore centerline in preparation of the boring operation. Photo by author. Cylinder Bore Distortion A look at static and dynamic cylinder bore shape issues. BY MIKE MAVRIGIAN Even with the best of intentions and insignifi cant, while in other situations, As noted by Sunnen, in order to preparation, an unavoidable degree of the change can be so dramatic as to accurately “map” a cylinder bore in cylinder bore distortion is likely to occur cause measurable ring dragging and order to obtain a clear dimensional under dynamic stress (heat and pressure). subsequent loss of power, fuel economy picture of how that bore is shaped from The challenge is to understand how and sealing due to both out-of-roundness top to bottom, a special “PAT” gauge is these changes take place and to establish and frictional heat. In some engines, used. This is an inclinometer that features procedures that will attempt to minimize these problems may be compounded if a shaft and probe. The shaft is affi xed these changes. As the block ages and/or is the specifi c block is severely affected by to the deck or torque plate and runs exposed to thermal changes, the casting’s vertically through the bore centerline. additional distortion that results from molecular structure will change, however The probe runs along the shaft vertically clamping forces caused by bell housing slightly, which will affect bore geometry. and monitors the bore walls radially. In addition, distinct changes in bore bolts, water pump bolts, motor mount This provides a dimensional perspective shape will take place as the block is bolts, etc. view of the entire bore, relative to assembled. The clamping forces that Granted, only the more severe cases the theoretical bore centerline. Don’t result from the installation of cylinder will be a cause of concern on a street expect to run out and buy one of these, heads will, in most cases, cause the engine. However, if the goal is to produce as they cost around $175,000 and are bores to “morph” as the fasteners an optimized racing engine, every factor primarily used by piston ring and honing pull and squeeze metal adjacent to the which can affect dynamic ring shape equipment manufacturers for analysis bores. Sometimes these changes are must be considered. applications. This allows plotting the 42 JULY-SEPT 2010 engine professional CYLINDER BORE DISTORTION BY MIKE MAVRIGIAN This stress graph illustrates how a cylinder bore may be distorted This dimensional “isometric” view further illustrates static from round to out-of-round as a result of head fastener loads. dimensional shift that can occur from cylinder head installation. (All illustrations provided courtesy of MAHLE Clevite.) actual shape of the bore in addition As mentioned earlier, when measuring Mahle-Clevite’s industry-respected to bore diameter. The readings can be a bore for shape, we need to remember Bill McKnight was kind enough to share displayed radially (viewing the bore from that, depending on the instrumentation bore distortion information that was overhead) to show where the bore shifts being used, we may or may not obtain a part of a previous Dana/Victor/Clevite from the centerline; and in an isometric true bore shape. If we use a bore gauge, NASCAR engine builders’ seminar view (side angle perspective in a variety we are simply measuring bore diameter presented by Victor Reinz engineer of view angles) that allows you to see the (and out of round) in specifi c height Ernest Oxenkonect, which addressed this entire bore in a dimensional manner. levels of the bore, but since we are not very topic. Selected excerpts from that In order to obtain bore diameter referencing from the true bore centerline, seminar follow. readings in your shop, without the use of we may be overlooking shifts of the Primary causes of bore distortion this sophisticated equipment, the cylinder centerline at various height locations. If in relation to the cylinder head/block walls can be measured with a bore gauge in doubt, or if we know that the radius joint include clamping load, cylinder at four different levels at four clock of the bore has shifted relative to the head gasket/combustion seal design, positions (12, 3, 6 and 9 o’clock). Once centerline, we can use a precision bore- the honing procedure and assembly these numbers have been recorded, place truing fi xture to correct the problem (such procedures. a honing plate (one per deck) on the as those offered by BHJ, CWT, etc.), or block, torque the plate(s), and re-measure take advantage of CNC technology in Clamp Load the same points to reveal differences that order to create a “new” bore centerline Bore distortion is generally directly based on blueprint data from the block related to clamp load. The clamp load have occurred. However, bear in mind manufacturer. The bores can then be must restrain combustion lift-off force that this will not reveal concentric bore “clean-bored” to an oversize, with the and still provide adequate force for the diameter shifts relative to the centerline. cutters referenced from a fi xed (and gasket to do its job. Additional clamp Another variable relating to bore presumably correct) centerline. This will load generally only leads to additional distortion is the cylinder head itself. After create bore trueness in a static condition, bore distortion. In other words, measuring the bores (accessing from but subsequent changes may still occur tightening head fasteners beyond the the bottom of the bores) on a relaxed when the block is subjected to loads, recommended specifi cation will only block (no heads or torque plate), install pressures and temperature. hurt. The optimal goal is to minimize and torque a cylinder head, and read Keep in mind that when we’re dealing clamp load and distribute the clamp load the bores again from the bottom of the with cast iron blocks, the very nature of uniformly while maintaining combustion bore to note the changes that have taken the casting process and the material mix and fl uid seals. place. Next, remove that head and install can and will create differences from block Distribution of clamp load affects a different head of the same type and to block. In other words, if you lined up bore distortion. An equal load measure the bores once again. If you fi nd fi ve blocks with identical casting numbers distribution is the goal. a different distortion level/pattern, don’t and identical age, you’ll probably fi nd One interesting item to note is be surprised. Depending on the makeup fi ve different variations of bore distortion. that the end cylinders are typically of the cylinder head, the clamping forces Knowing this, we need to temper our over-loaded, as they aren’t required to may reveal a different situation due to view of block analysis. Just because one share bolt load between cylinders. For the structure of the head, especially on small block Chevy of a particular series custom racing applications, consider cast heads, due to differences in the hard/ and casting shows unruly bore distortion smaller diameter studs/bolts for deck- soft internal makeup of the casting core. in number 3 bore, this does not mean that end positions to reduce the tendency of If the head pulls down harder or softer in this condition will be exactly repeated in over-loading the end cylinder areas. An various areas, this will accordingly affect every block of that vintage and type. Each excellent method of checking this is to how the block is stressed, resulting in block casting is its own animal and needs perform pressure fi lm studies (carbonless variations of cylinder bore shape. to be treated as such. impression paper) to inspect head gasket 44 JULY-SEPT 2010 engine professional 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. Photo by author. It’s common practice for builders to install a deck plate prior to honing. However, it’s more desirable to install two deck plates (one at each bank) to more closely simulate the stresses that the block will incur when it is assembled. Photo by author. engine professional JULY-SEPT 2010 45 CYLINDER BORE DISTORTION BY MIKE MAVRIGIAN load distribution, in an effort to equalize loads by possibly using smaller-diameter fasteners at end positions. Bolt clamp load can be measured by referencing bolt stretch (in terms of length). This is done with a Bidwell Bolt Gauge (formerly known as a Raymond Bolt Gauge), which sends an ultrasonic wave signal through the bolt from the head (or top of the stud). Once this signal travels down to the bottom of the bolt, it then shoots back to the signal sensor. The amount of time that is takes for signal return determines length, or amount of stretch. The bolt is fi rst measured Even with the use of a statically to create a reference value, deck plate, although a statically-round which is then compared to stretched bore may be achieved, length under installed conditions. The cylinder wall profi le fi nal value can be measured in terms of may change when pounds of clamping load. For example, subjected to operational if a 7/16” bolt is rated at 10,000 lbs and heat and pressure. the installed reading shows a clamping Photo by author. load of 12,000 lbs, it’s obvious that the bolt has been stretched beyond its clamp load design. Also, bolt load can be tailored to achieve a uniform distribution sealing stress in the combustion area of clamp load across all areas of the Honing Assembly Procedure of 7500 psi.