Section II. ROTATING AND RECIPROCATING PARTS
reason, it has been found that aluminum is the
best material for piston construction. It has a very
high strength-to-weight ratio. In addition to
a. General (Fig. 3-12). The piston is the part
being lightweight, aluminum is an excellent
of the engine that receives the energy from the
conductor of heat and is machined easily. Pistons
combustion and transmits it to the crankshaft.
also are manufactured from cast iron. Cast iron
also is an excellent material for pistons in low-
b. Requirements (Fig. 3-12). The piston
speed engines. It is not suitable for high speeds
must withstand incredible punishment under se-
because it is a very heavy material.
vere temperature extremes. The following are
examples of conditions that a piston must with-
d. Controlling Expansion (Fig. 3-13). Pis-
stand at normal highway speeds.
tons must have features built into them to help
them control expansion. Without these features,
(1) As the piston moves from the top of the
pistons would fit loosely in the cylinders when
cylinder to the bottom (or vice versa), it ac-
cold, and then bind in the cylinders as they warm
celerates from a stop to a speed of approximately
up. This is a problem with aluminum, because it
50 mph (80 km/h) at midpoint, and then deceler-
expands so much. To control expansion, pistons
ates to a stop again. It does this approximately 80
may be designed with the following features:
times per second.
(1) It is obvious that the crown of the piston
(2) The piston is subjected to pressures on
will get hotter than the rest of the piston. To
its head in excess of 1000 psi (6895
prevent it from expanding to a larger size than the
rest of the piston, it is machined to a diameter
that is approximately 0.03 to 0.04 in. (0.762 to
(3) The piston head is subjected to temper-
1.016 mm) smaller than the skirt area.
atures well over 600F (3160C).
(2) One of the ways to control expansion in
c. Construction Materials. When designing
the skirt area is to cut a slot up the side of the skirt.
pistons, weight is a major consideration. This is
As a split-skirt piston warms up, the split will
because of the tremendous inertial forces created
merely close up, thereby keeping the skirt from
by the rapid change in piston direction. For this
expanding outward and binding the piston in the
(3) Another variation of the split-skirt pis-
ton is the T-slot piston. The T-slot piston is sim-
ilar to the split-skirt piston with the addition of a
horizontal slot that retards heat transfer from the
piston head to the piston skirt.
(4) Some aluminum pistons have steel
braces cast into them to control
e. Cam Grinding (Fig. 3-14). By making the
piston egg-shaped, it will be able to fit the cylinder
better throughout its operational temperature
range. A piston of this configuration is called a
cam-ground piston. Cam-ground pistons are
machined so that their diameter is smaller parallel
to the piston pin axis than it is perpendicular to it.
When the piston is cold, it will be big enough
Figure 3-12. Piston