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Buy with confidence! If for any reason you're not completely satisfied with an item, simply return it within 7 days and the purchase price will be refunded.
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We ship world wide. All international orders must be paid online. Checks or money orders drawn on non-US banks will not be accepted.
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Choosing the Right Piston
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Aluminum pistons are really rather amazing. They are alternately seared by the heat of combustion, and then blasted by a jet of cold air with every intake stroke. They are accelerated and decelerated at tremendous speed with every turn of the crankshaft, while withstanding side loads that try to weld the piston skirts to the cylinder walls. Everything that an engine builder does to increase performance, from installing a hot-rod cam to porting the heads to bolting on a blower, is intended to do just one thing: increase cylinder pressure. And, of course, the more pressure there is in the cylinders, the higher the loads the pistons must endure.
Much of the work devoted to piston preparation is required by the special characteristics of aluminum. Since aluminum expands at approximately twice the rate of steel, clearances that are correct at room temperature can change dramatically when the piston tops are heated to over 500 F degrees. Forged aluminum pistons have a much denser molecular structure than cast pistons, so heat transfers through the forged material more quickly. Also, different parts of the piston expand at different rates. There is more metal around the pin bosses than in the skirts, so these two areas grow to different sizes when heated. This is why an engine that is noisy when first started on a cold morning can run quietly after it has warmed up. As the pistons are brought up to operating temperature, they expand to create the proper clearances.
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This diagram illustrates the heat distribution in cast and forged aluminum pistons. The densely packed molecules in the forging allow it to conduct heat away from the piston top quickly.
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Piston-to-Wall Clearance The right clearance between the piston skirts and the cylinder walls is absolutely essential. If the piston-to-wall clearance is too large, the pistons rock back and forth in the bore. This prevents the rings from sealing the cylinder properly, and can crack the skirts as the piston slaps from side to side. If the clearance is too small, the piston will literally stick in the bore, scuffing the skirts and destroying the wall finish. There’s a thin line between too much and not enough, but providing the correct running clearance pays dividends in both durability and horsepower.
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Piston skirts are slightly elliptical when measured at room temperature. This oval shape is the piston’s “cam grind.”
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A “barrel face” piston has a skirt that bulges outward in the center. Because of this complex shape, it is essential to follow the piston manufacturer’s instructions when setting the piston-to- wall clearance.
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There are two piston characteristics that affect the clearance. The first is the cam grind. All good quality pistons have skirts that are not perfectly round. If you were to cut a piston in half horizontally through its pin bore and then examine the skirts, you would discover that the skirts are slightly elliptical or oval. The eccentricity is hardly noticeable—usually between 0.020- and 0.040-inch—but it’s important. The skirt diameter perpendicular to the pin bore is the widest part of the piston, becoming smaller as you move around toward the pin axis. Why? The answer again lies in the different expansion rates of various parts of the piston.
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Proper piston-to-wall clearance is essential to protect the skirts from scuffing. Measure the piston diameter at the point specified by the manufacturer.
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Inserting a feeler gauge between the skirt and the cylinder bore can also check the piston-to-wall clearance.
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The second piston characteristic that must be considered is taper. If you measure that skirt diameter at several places between the top and bottom, you will probably discover that the average piston is wider at the bottom of the skirt tangs than it is at the piston pin hole. The amount of taper is very slight, typically between 0.005- and 0.010-inch.
Since the range of piston-to-wall clearances for high-performance pistons runs from 0.0015-inch all the way up to 0.015-inch, piston taper can have a tremendous impact on the actual clearance. Some pistons have an even more complex skirt shape called a barrel grind. Barrel ground pistons have skirts that bulge outward beneath the oil ring groove, then taper back inward as you move toward the tangs at the bottom of the piston. Regardless of how the piston is shaped, it is extremely important to know where on the skirt the manufacturer wants you to measure the clearance!
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Most piston makers call for measuring the piston diameter perpendicular to the wrist pin when setting the piston-to-wall clearance. There are exceptions, however. Some racing pistons are measured just below the oil ring groove, while others are miked at the tangs. These differences depend on the particular cam and taper that the manufacturer has chosen.
Other factors also affect the clearance. There are several different aluminum alloys commonly used by piston manufacturers. Although aluminum is the primary ingredient, the amounts of silicon, magnesium, manganese, nickel, copper, and other elements vary between the different alloys. These differences in turn affect the expansion characteristics and wear properties of the piston.
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The design of the piston skirts also must be considered. Stock cast aluminum pistons, for example, often have steel struts inserted in the pin bosses. These struts hold the skirts in a permanently expanded position, so the piston-to-wall clearance can be greatly reduced. Also, slots behind the oil rings in these pistons effectively isolate the piston head from the skirts, creating a heat dam that prevents the transfer of heat below the ring band. These features produce a very quiet engine, which is ideal for an everyday commuter motor. Forged pistons, in contrast, do not have struts, so the piston-to-wall clearance must compensate for thermal expansion. Also, high-performance pistons frequently rely on holes drilled in the back of the bottom ring groove (instead of a wide slot) to drain oil away from the cylinder walls. Thus, the heat at the top of the piston can travel downward to the skirts, where it causes the aluminum to expand.
Engine speed also determines the optimum piston clearance. High-rpm racing engines require more piston-to-wall clearance than street motors. Piston noise is not a consideration when you are running with wide-open exhaust headers, so racing engines have decidedly looser piston clearances to prevent skirt scuffing under the heat and pressure of competition.
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With so many variables to consider, the best advice is simply to follow the piston manufacturer’s recommendations—precisely! The piston maker will be familiar with the expansion characteristics of their piston material and skirt design. Measure the piston diameter exactly where the instructions tell you to take the readings. And don’t talk yourself into believing that a “little extra” skirt clearance is a good thing. More clearance than is necessary simply makes the engine.
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Piston Materials and Applications Cast pistons have been the standard automotive pistons for decades. They are inexpensive and easy to produce. They have a thermally stable crystalline grain structure and often incorporate cast-in steel expansion struts that allow them to fit tightly in the bore for optimum stability and ring seal. Under normal use they will stand up well to tens of thousands of miles of use. However, they have limited speed, thermal, and detonation resistance. They should only be used in moderate performance engines where speed is limited and detonation is strictly avoided.
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Hypereutectic pistons are castings, but they have nearly 2-1/2 times the silicon of a standard cast piston for increased hardness and greater resistance to higher temperatures and cylinder pressures. They are dimensionally stable and require very little skirt clearance. In some instances they can be operated with less clearance than standard cast pistons. This feature keeps the pistons and ring package well stabilized in the bore and improves sealing and blowby control. While Hypereutectic pistons are well suited to street performance applications, they do not have the detonation and temperature resistance of forged pistons. They should not be used with more than very light nitrous-oxide injection loads, nor with high-pressure turbo- or supercharging systems.
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Forged pistons are manufactured with a forging dye from a solid slug of heat-treated aluminum alloy. They possess the dense grain structure and metallurgical properties to stand up to severe use, including a degree of detonation resistance. However, forged pistons have less dimensional stability and require greater skirt clearance for reliable operation. Forged pistons remain the top choice when strength and durability are required for racing, turbocharged, supercharged, or nitrous-oxide-injected applications.
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Previous | Next
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This has been a sample page from
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The Step-By-Step Guide to Engine Blueprinting by Rick Voegelin
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Practical Methods for Racing and Rebuilding How to buy machine shop work Selecting and preparing parts
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This completely revised and updated version containing an additional 32 pages is simply the best book you can buy on engine preparation for street or racing! Rick Voegelin's highly acclaimed combination of savvy writing and wrenching skills puts this best-seller in a class by itself. All important preparation techniques are clearly illustrated and explained in this easy-to- read text. Engine Blueprinting shows the reader how to use precision measuring tools, calculate compression ratios, degree a camshaft, and much more! Loaded with helpful advice, this book should be in every enthusiast's tool box.
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Click below to view sample pages from each chapter.
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"Rick Voegelin's book, The Step-by-Step Guide to Engine Blueprinting, is an excellent source of performance-oriented engine building information for the beginner and the seasoned veteran alike. This digest should be in every enthusiast's greasy mitts."-- Steve Magnante, HOT ROD
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Chap. 1 - Engine Blueprinting Chap. 2 - Cylinder Block Chap. 3 - Crankshaft Chap. 4 - Connecting Rods Chap. 5 - Pistons Chap. 6 - Cylinder Heads Chap. 7 - Camshaft Chap. 8 - Compression Ratio Chap. 9 - Balancing Chap. 10 - Assembly Tips
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Softbound 8-3/8 x 10-7/8 160 pages 400 b/w photos Item #SA21 Price: $18.95
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Click here to buy now!
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Shipping is combined and discounted for multiple item purchases! Buy more and save on shipping! We ship Worldwide! See International Shipping for more information!
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MRE PO Box 47 Grinnell, IA 50112
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