'Hang around a bunch of motorheads during a bench-racing session and within the first five minutes you're nearly guaranteed to hear someone claim to have an engine that's been "balanced and blueprinted." It's one of those terms that's been thrown around since your grandpa was tuning on his Model T, and yet few can explain exactly what it means. Despite what you'd believe based on the frequency of its usage, the label of balanced and blueprinted actually refers to an engine treated to far more than a simple rebuild.
The basic premise is that the production engines we tinker with were designed to be produced on an assembly line by an army of automated machines and fast-moving technicians, and as such, some flexibility in specifications has to be tolerated by manufacturers to allow for variances in quality. This flexibility can be seen on engine-specification charts in areas like combustion-chamber volume and piston-ring endgap where proper measurements are given in ranges rather than pinpointed figures. The original equipment (OE) manufacturers knew their engines would run for years without trouble in whatever vehicle they were installed, even if every spec were on the less favorable side of the scale, but that's where they left room for improvement.
Since you don't have to be as lenient when building your engine, you have the option of making all specifications exactly what the engineers thought they should be. This means each combustion chamber can be manipulated to displace the smallest allowable volume, each piston ring can show the minimum amount of gap, and each bearing can hug its journal at the tightest tolerance recommended. In addition, all of the moving parts in the bottom end can be weight-matched, and then the rotating assembly can be spin-balanced to remove any hint of vibration.
In theory (and usually in practice), such attention to detail should translate into increased power and durability, but you still have to know what deserves attention and how to determine if it is in fact exactly where it should be. For that we've assembled a collection of the primary areas that should be scrutinized during your next engine build, and what you should do about determining and assuring the accuracy of each spec. Some of these items require specialized equipment most of us may never actually own, but the information is still valid, as it will allow you to communicate with and pester your machinist to get the results you're looking for. Read over our advice, and if you're getting ready to build, follow it up with the further reading we've recommended
How to Read a MicrometerIf you're going to blueprint an engine, you'll need to learn to read a micrometer. You could use an electronic digital mic, but a real gearhead should know how to read an old-school micrometer. Here's how to read a mic accurate to 0.0001 inch.
Start by verifying that the mic reads zero when the spindle closes to the anvil. If it doesn't read zero, adjust the mic until that is achieved. If you look on the spindle, the long horizontal line should line up with the zero line on the rotating barrel. The large hash marks along the long horizontal line with numbers represent 0.100 inch (0.100, 0.200, and so on). Within each of the large numbers are three smaller, vertical hash marks on the horizontal line that represent 0.0250 inch. Around the rotating barrel is a series of numbers from 0 to 24. This means that each full rotation of the barrel equals 0.025 inch. Finally, you'll see a series of long parallel lines above the main horizontal line that break down 0 to 9. These lines represent ten-thousandths of an inch (0.0001 inch).
In our example, the micrometer is set at 1.3775. Let's see how we get there. First of all, we're working with a 1- to 2-inch micrometer. Next, note that the rotating barrel has uncovered the main horizontal line well past the 3. Next, we start counting the 0.025-inch hash marks, coming up with three for 0.075 inch. That means we're at 1.3750. Now let's look at the marks on the rotating barrel. Note that the main horizontal line indicates the position between the 2 and the 3, so add 0.002 + 1.3750 = 1.3770. If the main horizontal line had lined up directly with the 2, we'd be done, but since it doesn't, take a look at the lines above the main horizontal line. Here, the 5 lines up perfectly, indicating we should add an additional 0.0005 inch to our reading-0.0005 + 1.3770 = 1.3775. There you have it. Now you know how to read a mic.
Crank endplayCrank endplay is a measure of how far the crankshaft can travel fore and aft in the engine once it is installed, which is determined by the amount of clearance between the thrust bearing and the thrust flange or thrust journal, depending on the engine design. A general guideline for thrust clearance is somewhere between 0.005 and 0.0010 inch, but as always, the exact spec for your engine should be referenced.
With most engines, setting the thrust bearing is a recommended practice during assembly. This typically involves setting the crank in place with the main caps installed but not yet tightened. The crank is then struck with a soft-faced mallet on each end to shift it fore and aft, aligning the two thrust-bearing shells.
Once the thrust is set, the clearance can be measured. On some engines, feeler gauges can be used to determine the amount of thrust, but most engine builders prefer to use a dial indicator with a magnetic base. The dial indicator should be assembled so that the plunger is parallel with the crank's axis; a prybar is then used to shift the crank back and forth in the block. If the dial is set to zero with the crank at one extreme of its thrust travel, when the crank is shifted to the other extreme, the reading should indicate total endplay.
It's fairly common when using worn or even reconditioned crankshafts to find excessive endplay, and correcting this can be difficult. As with rod side-clearance, a little extra endplay can be allowed, but if the crank's thrust movement is too great to ignore, it may have to be repaired or just replaced. If there is too little thrust clearance, the thrust bearings can be sanded to increase the clearance. The recommended procedure for this is to use wet/dry sandpaper with a grit of around 1,000. Using a flat surface under the paper, the bearing is sanded carefully and then reinstalled to check the increase in clearance. This usually will require several test fits, and possibly some initial work with a more coarse grit if the clearance is particularly tight.
Measuring Bearing ClearancesThe most important fact that many enthusiasts miss when measuring bearing clearance is that the housing bore is a big factor in setting clearances. Bearing shells don't vary by more than 0.0001 inch in thickness, but housing bore diameters can easily vary by 0.0005 inch. The simple, easy way is to use Plastigage, but to establish a true bearing clearance, you need to use a micrometer and measure everything.
Let's deal with a connecting rod and rod journal. Start by measuring the connecting rod journal with a micrometer. For the sake of easy math, let's say the journal diameter is exactly 2.0000 inches. It's best to use a mic that indicates to the ten-thousandths of an inch (0.0001 inch). Next, place a set of rod bearings in the matching rod and torque the cap in place. Use a quality dial bore gauge to measure the inside diameter of the connecting rod in the vertical plane. A dial-bore gauge accurately reads the inside diameter of cylinder, main-bearing, or rod-bearing bore. There's a photo of a dial-bore gauge on page 42.
Let's say that the inside diameter of the rod bearings measure 2.0022 inch. Subtract the rod journal diameter from the bearing id and this will give you the true bearing clearance (2.0022 - 2.0000 = 0.0022 inch). Typical rod and main bearing clearances vary between 0.0020 and 0.0030 inch with the ideal clearance closer to 0.0020 inch. You can also try this trick: If you find you have both a tight and a loose clearance with a couple of rod bearings, try swapping the two and recheck the clearances. Often, they will fall more closely in line with the others.
You can also mix and match over- and under-size bearings to customize the clearances. Let's say you are using a set of standard main bearings and a tolerance stackup creates an overly tight clearance of 0.0017 inch. By adding one shell half of a +0.0010-inch oversize bearing set, this theoretically will add 0.0005 inch to the clearance. Often, this doesn't work out this precisely, but this is an accepted procedure as long as you're using under- or over-size bearings from the same manufacturer. A good rule of thumb for rod and main bearing clearance is 0.001 inch for every 1 inch of journal diameter and then add 0.0005 inch just to be safe. So for a 2.00-inch rod journal engine, this would be 0.0025 inch.
Computing Compression RatioLet's make this easy. There are at least two Web sites that offer free computer programs for determining static compression ratio. The two programs can be found at performancetrends.com and kb-silvolite.com which is the parent company for Keith Black pistons. These calculators only require you to input the proper data into the programs to generate the static compression ratios. Here's what you need to know.
Simply stated, compression ratio is the mathematical ratio between the volume of the cylinder with the piston at bottom dead center (BDC) divided by the volume of the same cylinder with the piston at top dead center (TDC). To compute these volumes, you need to know several variables that affect these volumes.
We'll start with cylinder bore and stroke. For our example, let's use a 454 Rat motor with a 4.250-inch bore and a 4.00-inch stroke. As bore increases in diameter, this will increase compression since we're dealing with a larger-diameter cylinder. Stroke has a big impact on compression as well for obvious reasons since this also increases or decreases the cylinder volume. These volumes are generally established in cubic inches.
Next let's deal with the head gasket. The thickness of the gasket creates a space between the head and the block, adding volume. This is usually expressed as the compressed thickness of the gasket, usually between 0.015 and 0.050 inch. If we want to split hairs, generally the gasket bore is larger than the diameter of the cylinder, adding to the gasket volume figure, but this is generally a very small value.
A similar value to head-gasket thickness is piston deck height. The farther the piston is down in the cylinder at TDC, the more combustion-area volume it creates. This reduces compression. Conversely, milling the block deck surface, reduces the piston deck clearance and increases compression. Generally, a zero deck height, where the piston top is equal to the block deck height, is considered ideal, especially for engines with wedge style combustion chambers.
Combustion-chamber volume also has a significant effect on the compression ratio. The same is true with piston-dome volumes. Obviously, as chamber volume decreases, compression will increase and vice versa. Pistons with a dish effectively increase combustion chamber size, while domed pistons reduce chamber volume. All these volumes are generally expressed in cubic centimeters (cc) and measured with a calibrated burette. The computer programs automatically convert cc's into cubic inches so that all the values are the same, but if you're doing the math on paper, the conversion is to multiply cc times 0.0610237 to equal cubic inches. For example, a 100cc chamber would be 6.10 cubic inches. Conversely, multiply cubic inches by 16.387 to get cubic centimeter.
The easiest way to measure piston dome or dish volume is to cc the piston in the cylinder. Seal the rings with grease, accurately place the piston 0.100 inch down in the cylinder and then measure the cc volume by filling up the cylinder. Next, compute the volume of a standard cylinder (bore x bore x height x 0.7854). For example, a 4.00-inch bore and a 0.100-inch height would be: 4 x 4 x 0.100 x 0.7854 = 1.256 ci x 16.387 = 20.59 cc. If you are measuring a piston with a dome, the measured volume will be less than the computed volume with the difference being the effective dome volume. For a dished piston, the measured volume will be more with the difference being the effective dish volume.
The real beauty of these computer programs is you can experiment with different values to quickly home in on a combination that will produce the ideal compression ratio. You can juggle combustion-chamber volume, deck height, head-gasket thickness, piston-dome (or dish) volume, and even bore and stroke and how these variables will affect compression ratio. Experiment for yourself-it's fun if you're into engine blueprinting to come up with the best engine combination.
Piston-to-wall clearanceSo you're having your engine bored 0.030-inch over and you order a set of 0.030-inch-over pistons, and then it hits you: If the bore is 4.030 inch and the piston is 4.030 inch, how will they fit together? The piston is not 4.030 inch, but rather, slightly smaller to create the desired fit between the piston and the bore, which should be right around the specified 4.030 inch. We say "around" because this is one of those areas where attention to detail will yield an optimum fit.
The first step is measuring the pistons that are to be used, but simply straddling one with a caliper is not accurate because pistons are not perfectly cylindrical despite what your eye may tell you. In reality, pistons are barrel shaped, having tapered ends that are actually narrower in diameter than the piston is in the middle. But the amount of taper and where it begins varies from manufacturer to manufacturer, and can vary among piston designs within the same manufacturer. For this reason it is critical that you consult with the manufacturer to determine the proper place to measure the piston at its widest point. This is usually located at a point just under the wristpin, but again-don't guess.
The next step is to measure the bore. In a machine shop, this will usually be done with a dial-bore gauge-a large, spring-loaded internal caliper with ball-bearing tips and a dial that can quickly determine the bore diameter. The machinist should measure prior to machining, and several times during the process as well. The critical measurements will be taken during the final hone, as this is where the piston-to-wall clearance will be determined. With the piston's actual diameter in mind, the machinist will carefully hone each bore to its final dimension, removing just enough material to obtain the desired clearance. So if the piston actually measures 4.026 inch, and the desired piston-to-wall clearance is 0.004 inch, the bore should be honed to 4.030 inch exactly. The actual piston-to-wall clearance will be recommended by the manufacturer and will vary depending on the type of piston; for example, forged pistons generally require more piston-to-wall clearance than hypereutectic because the forged units will experience more thermal expansion as they reach normal operating temp.
Fitting Rings to PistonsThis might seem like a simple operation-merely slip the rings on the pistons and you're done. But the reality is that attention to detail with regard to rings can mean a better quality seal when the pressure's on. You'll need to refer to each manufacturer's recommendations for each of these specifications since they may vary. Besides setting the ring endgap, the two critical dimensions are ring side and back clearance. Side clearance is the gap between the top or bottom of the ring to the ring groove while the back clearance prevents the ring from protruding out from the piston wall. Higher quality piston manufacturers spend more time ensuring that the machined ring grooves are parallel to improve ring seal while high-end ring companies spend loads of money ensuring that ring faces are as parallel as possible. Really anal engine builders prefer to lap their own rings on a perfectly flat surface using 1,000-grit wet/dry paper.
Ring endgap It's common knowledge that setting proper ring endgaps will improve cylinder sealing and therefore performance. Typical replacement and performance ring packages are designed for a given bore size, offering a sizeable endgap (especially for the top ring) to prevent butting the ends, which causes all sorts of mechanical grief. For the car crafter, most performance ring manufacturers offer 0.005-inch oversize ring packages that allow you to custom set your top and second-ring endgaps.
This task is not difficult, but can be time consuming. If your budget is tight, several companies offer manual ring grinders like one from powerhouseproducts.com (PN POW105050 $65.00). To measure the endgap, square the ring in the bore and measure the gap with a feeler gauge. Keep track of the number of turns on the ring grinder and work slowly until you establish the proper endgap. Then you can use the turn count to cut the remaining seven rings. Always trim just one side of the ring and make sure to keep that edge perpendicular. Don't forget to deburr the end once the gap is set. Always file toward the id of the ring to prevent peeling away the ring face material.
A good general rule for ring endgap is 0.0040 inch per inch of bore for a normally aspirated street engine on pump gas. Nitrous, super- or turbo-charged engines generally want more endgap. The other major move now is to open up the endgap for the second-ring package to prevent pressure buildup between the second and top rings that could unload the top ring.
Piston Compression HeightThis may be a bit esoteric, but useful information for those of you considering doing a stroker engine package. All engines incorporate what is called block deck height. This is the distance from the crank centerline to the top of the block deck where the head bolts on. There are four basic variables that go into determining the stack-up of components for this spec: the stroke, connecting-rod length, block deck height, and piston compression height. These are all self-explanatory with the exception of piston compression height. This is defined as the distance from the centerline of the wristpin to the flat portion of the piston that is parallel with the deck. This does not count any dome or recessed portions of the piston. The formula goes like this: piston compression height + rod length + half the stroke + the piston deck height should equal the block deck height
Let's take an easy example. Let's say we wanted to build a stroker small-block Chevy, with a block deck height of 9.025 inches. The stroke for a 383 is 3.75 inches, and let's say we wanted to run 6.00-inch rods. What's left is to figure out what compression-height piston to use. Half of the 3.75-inch stroke is 1.875, and adding 6.00 for rod length equals 7.875 inches. Assuming a 9.025-inch deck height, subtracting 7.875 from 9.025 equals 1.15 inches. This can be the piston compression height, but that doesn't leave any room for milling the block and assumes a zero piston deck height. In this case, we would look for a piston with a slightly shorter deck height to accommodate these needs. Luckily, the piston manufacturers have done most of this homework for you, but it still is worth knowing how it all fits together.
Rod Bolt StretchThe most highly stressed fasteners in any internal combustion engine are the connecting-rod bolts. While the troglodytes will continue to use a torque wrench and the classic "I've been doing it this way for 20 years!" excuse, the point is that there is only one correct way to torque rod bolts and that's by measuring rod-bolt stretch. All fasteners are designed to stretch when torqued. The problem is creating the proper torque to establish the ideal bolt stretch. Variables of lubrication and uncontrolled friction account for most of the torque applied to a bolt, the rest actually stretches and preloads the bolt to create a clamp load.
So the bolt manufacturers created a tool that only measures stretch. The beauty of a rod bolt is that both ends of the fastener are accessible to measure stretch, unlike a main cap or head bolt. In the case of ARP fasteners for a typical 31/48-inch rod bolt, the stretch figure is 0.0064 inch. That means you continue to tighten the fastener until the body of the bolt has stretched to that figure. That creates the maximum clamp load while still leaving approximately 25 percent of additional stretch to accommodate operating loads. Remember that as rpm increase, load increases geometrically. That alone should motivate you to never assemble another engine without measuring rod-bolt stretch.
Rod side-clearanceMeasuring rod side-clearance is a simple matter of inserting feeler gauges between assembled pairs of connecting rods to determine the amount of clearance between the rods and the sides-or "cheeks"-of the crank journal. Generally, on V-8s we're used to 0.010-0.020 inch as the norm for steel rods (more for aluminum rods), but as with most specs, the ideal amount can vary between manufacturers. The potential trouble here is that since the rods must be assembled to take this measurement, it is often first dealt with during final assembly, at which point finding too much clearance, which is quite possible when dealing with reconditioned cranks and rods, is a difficult situation to remedy. This is just one example of why it's a good practice to assemble the engine to measure specs prior to final machining if possible. The upside is most builders seem to feel that a little extra side-clearance is not a real problem. On the flip side, too little definitely can be. When assembling your rods, particularly during a mock-up phase when the pistons may not yet be mounted, make sure to keep the chamfered side of the rod facing out toward the crank's cheek. If the unchamfered side of the rod is installed against the radius between the journal and the cheek, it will likely push the rod toward the center, resulting in a false clearance reading; installing the rods this way during final assembly can cause the rotating assembly to bind, resulting in damage if the engine is started. Note that some engines don't have radiused rod journals and therefore may not have chamfered rods. Another tip: Don't measure side-clearance at only one point between pairs of rods-gauge the gap all the way around, as imperfections in machining can leave high spots on rod sides.
If you find too little rod side-clearance, the rods can be filed or sanded to provide more. Professional machinists will often use a belt sander, but this requires an experienced touch as it is easy to remove too much material. Some builders like flat files, others prefer emery cloth wrapped around pieces of flat steel. The objective is to keep the side of the rod flat and parallel to the beam.
Deck Height/QuenchThe difference between an engine assembler and an engine builder is little details like checking the relationship between the piston and the block deck. Generally, most domestic engines place the flat portion of the piston below the block deck for piston-to-head clearance, often as much as 0.050 to 0.080 inch down into the bore. If improved combustion efficiency and more torque and power are your goal, a good way to get there with a wedge-type (non-Hemi) combustion chamber is to tighten the quench, which is the area between the flat portion of the cylinder head and the top of the piston. For steel rod engines, this can be as tight as 0.040 inch or even less if you're brave. This clearance also includes the head-gasket thickness. A tight quench area effectively squishes air and fuel into the chamber as the piston approaches TDC, improving combustion activity and generating a more efficient combustion process. This also reduces detonation sensitivity.
The best way to measure piston deck height is to mock up the crank, connecting rods, bearings, and pistons (without rings) and place each piston at TDC to measure the difference between piston and the deck. Bring the piston up to TDC and then use a deck-height mic or a deck bridge and dial indicator to measure the differential. Measure all eight pistons to get an accurate map of how much the deck surface will need to be milled to create your ideal deck height.
All The RestThis blueprinting guide is hardly complete, especially when you consider that Rick Voegelin has written a much more extensive guide, The Step-By-Step Guide To Engine Blueprinting from SA Design, that has recently been updated. We've purposely skipped topics such as blueprinting oil-pump clearances, assembling cylinder heads, pushrod length, port-matching, and degreeing a cam. Many of these items have been covered in past Car Craft issues, but if there is enough demand we can certainly present the information with all the latest updates. Let us know what you want to see.