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. Lift-off force for a 4.25” deck and head gasket by experimenting Install a deck plate (torque plate) and bore @ 1400 psi peak pressure is19,861 with monitored bolt loads while using a head gasket to initial assembly load. Next, relax the clamp load to levels lbs. Depending on the application and telltale load indicator such as carbonless actually measured from a conditioned number of bolts featured to seal each impression paper. Determining bolt load engine. Hone the cylinders at this point. bore, individual bolt load requirements knowledge is a critical factor and should Assemble the engine to exact initial may be in the 6165 – 11,917 lbs range. be gathered on engines both during clamp load of the data-bearing engine. For example, a typical ARP 7/16” stud assembly and during disassembly. Torque values are not adequate may be rated at 12,700 – 15,600 lb load Note: By the way, if you’re interested when performing assembly where range. in obtaining a Bidwell bolt gauge, optimal performance is required, and contact American Bolt Tightening at where exacting bolt/stud load must be Head Gasket Selection and 734-591-2055. The units run around accurately measured. Torque and torque/ Consideration $20,000, so while they’re not for every angle methods can result in 25 – 30% Wire ring style head gaskets may relax shop, operations building $30,000-and- load variations. Instead, measure bolt/ as much as 10 – 25% after initial up race motors may want to consider stud load with an ultrasonic gauge. assembly. By contrast, MLS gaskets relax the investment. If you plan to achieve Simulating operating cylinder head less than 10% after initial assembly, optimum cylinder head clamping and load on bores while honing is the thereby providing additional assurance minimal bore distortion by precisely most critical factor in minimizing bore of maintaining proper clamping load and verifying bolt load, it’s a must-have piece distortion. Again, this means going load distribution. of equipment. I had the opportunity to beyond merely using a deck plate with Wire ring style gaskets require the watch one of these gauges in use during bolts torqued to value. Actual bolt loads highest clamp load and can feature a race engine build seminar at the Dana must be monitored and achieved at pre- high peak loading along the cylinder school two years ago. The gauge is determined levels. openings. Again, by contrast, all active easy to use and is extremely accurate (it (spring function) MLS gaskets have the should be, considering the price). There’s How much is enough? lowest peak loading along the cylinder nothing wrong with a torque wrench Minimum clamp load must be greater openings. MLS gaskets with a built- when used for intended applications, than the combination of lift-off force and in “stopper” layer or beaded spacer but at the highest level of race engine required sealing force. provide high combustion sealing stress building, you really need to know what A general approximation (a good with lower required bolt loading. Active the bolt is doing internally when it’s general model) for clamp load is 3x’s lift- MLS gaskets are designed with formed securing the cylinder head to the block. off force. This generally results in static metal layers featuring contact and air

46 JULY-SEPT 2010 engine professional Three views of a “typical” dynamic bore distortion scenario. This graph illustrates how cylinder concentricity can be affected by the head clamping operation with regard to temperature and operation with regard to wall temperature and gas pressure.

These isometric views show bore shape changes This example illustrates a “before & after” axial view relative to temperature shifts. relative to hot honing. spaces that work as a spring to provide Thermal considerations and cold can be twice that of a torque compressive tension that aids in sealing during honing plate, or more. Because of the complex while requiring lower loading. Active Another aspect of obtaining a bore that interaction between engine design, MLS gaskets feature an elastic element will remain as true as possible during casting processes, uneven wall thickness, which absorbs cylinder head motion. engine operation involves heat. Consider variations in metal composition and Stopper MLS gaskets also feature a layer that while a bore may be correctively internal metal stresses, dimensional of “dead stop” that provides a limit to trued while the block is cold, it stands to changes of the cylinders within a block compression. Stopper type gaskets feature reason that bore shape may be affected can vary signifi cantly. Not only are there rigid combustion seals which bend the thermal dimensional changes, but these as temperatures rise and fl uctuate. In cylinder head over the seal as a result of changes are far from uniform throughout that regard, does it make sense to hone clamping loads. Active MLS offers low the bore. The major effect is the gross cylinders at room temperature, or at hardware distortion, since the gasket dimensional change in bore diameter, an elevated temperature that more “gives” to reduce the chance of distorting which can change .0005 - .001” of bores and head decks. This type also closely resembles levels experienced in an inch, per inch of bore diameter, at provides good recovery for lightweight a running engine? Some argue that the temperatures between 65°F and 210°F. hardware. However, it may be diffi cult change is minimal; or that the act of For a common 4” bore, this equals .002 to generate high combustion sealing honing generates surface heat on the – .004” of growth. stress with an active type MLS. The walls anyway. However, others are of The rest of these variations within the stopper MLS type of gasket provides high the opinion that the block should be bore…thermal distortions…. are largely sealing potential, but has the potential heated with circulating water in order non-uniform and can create critical to generate high hardware distortion. to replicate the effects of hot coolant concerns. These distortions include the Determining which type of gasket to use fl owing through the block (and adjacent local diametric variations in the walls, will be based on research data gathered to the bores). the barrel or hourglass effect, bores during engine components, engine test While a torque plate affects the upper departing from circular and becoming assembly and engine operation (on dyno 1.5” or so of the bore, the dimensional non-round and the bowing or arcing of and/or on-track). change of an average bore between hot the bore from a true cylinder. A normal

engine professional JULY-SEPT 2010 47 CylinDer Bore Distortion BY MIke MAVrIGIAN

bore gauge can detect all but the last of these distortions. Since the cylinder walls are not of uniform thickness and the cylinder is captured at each end, thermal expansion of the metal can and does cause distortion of the cylinder walls. A normal cylinder wall will assume a slight barrel shape as the temperature increases, with high and low spots that vary from cylinder to cylinder, causing a non-circular, non-uniform bore as the engine heats. Temperature is not the only effect on a bore. Under normal conditions, the engine cooling system operates at 10 – 14 psi. Normal engine pressure forces the cylinder into an hourglass shape. While 10 – 14 psi may not seem like much, it compresses the cylinder walls in ABOVE: The hot honing unit is plumbed to the block on the honing machine. the center around .0005”. The normal The theory is to attempt to simulate engine operating temperature (at least pressure effect from the cooling system in terms of coolant temp) during honing. RIGHT: Sunnen’s hot honing water tends to partially offset the barrel effect heater/delivery unit. Photos courtesy of Sunnen. of this thermal distortion. Because of these offsetting effects, hot honing must be done with the block pressurized to Aftermarket blocks (all materials): ambient temperature of 65°F is between achieve the best possible true bore. • Some are just copies of OEM and 0.003 - 0.004”. This anticipated change While a few attempts at creating a suffer the same ills is not necessarily linear, as different “hot honing” environment have been • Some are far superior in most of the areas of the cylinder bore may change by seen in previous years, an excellent above areas. varying amounts. example of a viable system is KW According to research done by KW, in Products’ “Hot Hone 2000.” This is a Torque plates reproduce the stress addition to the amount of average bore completely self-contained system that in the upper 0.5 – 1.5” of the bore size increase, the average difference from features a stainless steel cartridge pump, that would otherwise be produced by cylindrical in the heated and pressurized brass tank and armored safety hoses bolting the cylinder head in place while bore seems to be around .0015”. This with quick (“Kwik”) connect couplings. still allowing access to machining the change can be several times the change The system includes a fully adjustable bore. The actual measured amount of seen with torque plates. These are electronic thermostat, pressure and distortion in the areas around the bolt average fi gures and the real revelation temperature gauge, automotive-style at the top of the block is seldom more will be those areas of the bore that move fi ller, soft plug and block adapters for the than .0005 - .001”. This is however a outside of these norms without obvious small block Chevy. According to KW, the signifi cant amount when you consider reason. unit is adaptable to all honing machines that the average high-performance engine Theoretically, with the engine block and uses standard torque plates. Block builder won’t accept a bore that is less heated to operating temperature during adapters for other engines are also than .0002” straight and true. the honing process, the bores should available or can be fabricated by an It is usually necessary, especially with remain fairly stable and stay on-size individual shop. plain-Jane OE blocks, to torque-plate throughout the process.■ the block when you bore it if you plan to Various factors can affect use torque plates when honing. The hone cylinder bore geometry changes may jerk from the distortion if it wasn’t Factory original cast blocks: originally bored with torque plates and • The density of the material varies sometimes the bore is distorted enough • Core thickness varies throughout that by the time the machinist straightens • Machining tolerances are not within the bore, it may be larger than desired. acceptable performance guidelines When a block is heated to its • Webs or reinforcement of critical areas intended operating temperature and are not optimal the subsequent related pressure, the amount of distortion will vary depending • All core sand, ties, stands and Mike Mavrigian has written thousands of on many factors. An obvious few are machining debris is not removed technical articles over the past 30 years for a • Galleys are not sized or routed temperature, pressure, block material, variety of automotive publications. In addition, optimally varying density of material, quality of Mike has written many books for HP Books. • Water jackets and passages do not casting, Siamese bores, sleeved blocks Contact him at Birchwood Automotive Group allow for even heat transfer and offset boring. Creston, OH 44217-9527. Phone: (330) 435-6347. • Flow problems in molds create The average change in size of a E-mail: [email protected] weakened areas cylinder bore heated to 190°F from an Website: www.birchwoodautomotive.com

48 JULY-SEPT 2010 engine professional