tuffnuff
Moderator
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| Joined: 12/09
Posted: 01/16/13
12:15 PM
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PISTONS
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.
PISTON-TO-WALL
CLEARANCE
The right clearance between the
piston skirts and the cylinder walls is
absolutely essential. If the piston-towall
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.
Piston skirts are slightly elliptical when
measured at room temperature. This
oval shape is the piston’s “cam grind.”
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.
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 notice-
able—usually between 0.020-and
0.040-inch—but it’s important.
Piston Materials and Application.
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 expan-
sion 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 deto-
nation resistance.They should only be used
in moderate performance engines where speed is limited and detonation is strictly
avoided.
Hypereutectic pistons are cast-
ings, but they have nearly 2-1/2
times the silicon of a standard
cast piston for increased hard-
ness and greater resistance to
higher temperatures and cylin-
der pressures. They are dimen-
sionally stable and require very
little skirt clearance. In some
instances they can be operat-
ed with less clearance than
standard cast pistons. This fea-
ture 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.
Forged pistons are manufactured with a
forging dye from a solid slug of heat-
treated aluminum alloy. They possess
the dense grain structure and metallur-
gical 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 oper-
ation. Forged pistons remain the top
choice when strength and durability
are required for racing, turbocharged,
supercharged, or nitrous-oxide-injected
applications.
When The Flag Drops,,,
The Bull Chit Stops,,,
P. Engineer, Engine Builder
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Posted: 01/17/13
10:57 PM
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