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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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How to Rebuild the Small-Block Ford
This 144 page book guides you step by step through a rebuild,
including: planning, disassembly and inspection, choosing the
right parts, machine work, assembling your engine, first firing and
break-in. It also gives you helpful hints and tips on performance
upgrades, including cams, heads, ignition, induction, and more. It
also points out problem areas to watch for, professional builder
tips, jobs that need special care or special tools, and more.
Includes 495 color photos and covers the Ford 289, 302, 351W,
351C, 351M and 400.
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Price:
$22.95
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Turbochargers
How to select and install the correct turbo for big or small
horsepower gains. Discusses turbocharger design, sizing,
matching, controls, carburetion, exhaust, ignition,
intercooling, marine and high altitude applications. The most
comprehensive book available. Turbo suppliers and kit
maker addresses are included. “Everything you could possibly
need to know about turbochargers for automotive applications
is in this book.
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Price:
$18.95
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How to Build Big-Inch Ford Small Blocks
By increasing the bore and stroke of your current engine, you can
add those cubic inches without the hassle of switching to a big
block. George Reid thoroughly explains the building of a small
block Ford stroker, paying special attention to the effect that
increasing the bore and stroke have on the engine as a whole.
Also included is a complete guide to factory head and block
castings, as well as aftermarket block and head guides, so you
can choose exactly the right parts for your project.
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Price:
$18.95
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Payment, Shipping & Sales
Tax: Iowa
residents must pay 7% sales tax. Items usually ship
within one
business day of receipt of payment! Standard shipping is a flat rate of
$4.95 to
anywhere in the United States with USPS Media Mail.
Priority Mail shipping is available for an
additional $2.95, or
$7.90 for shipping. Shipping is combined and discounted for multiple item
purchases as follows: first item regular shipping price, add
$1.95 for each additional item. For
purchases of 3 or more items
shipping is automatically upgraded to Priority for no additional
charge! We offer world wide shipping and ship to Canada and Mexico
with USPS Priority Mail
International for $11.95, and to most
locations in Europe, Australia, Asia, Japan and South
America for
$14.95. Satisfaction is Guaranteed. Our store has a NO HASSLE RETURN
POLICY
within 7 days of purchase.
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