Fitting pistons into the cylinder properly is very
important. Because metal expands when heated, space
must be provided for lubricants between the pistons and
the cylinder walls. Pistons must have features built into
them to control expansion. Without these features,
pistons would fit loosely in the cylinders when cold, and
then bind in the cylinders, as they are warmed up. This
is the problem with aluminum because it expands so
much. The pistons (fig. 3-25) may be designed with the
following features to control expansion:
It is obvious that the crown of the piston gets
hotter than the rest of the piston. To 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 of an inch smaller
than the skirt area.
One way to control expansion in the skirt area is
to cut a slot up the side of the skirt. As a split-
skirt piston warms up, the split merely closes,
thereby keeping the skirt from expanding
outward and binding the piston in the cylinder.
Another variation of the split-skirt piston is the
T-slot piston. The T-slot piston is similar to the
split-skirt piston with the addition of a horizontal
slot that retards heat transfer from the piston
head to the piston skirt.
Some aluminum pistons have steel braces cast
into them to control expansion.
The skirt, or bottom part, of the piston runs much
cooler than the top; therefore, it does not require as
much clearance as the head.
The piston is kept in alignment by the skirt, which is
usually CAM-GROUND (elliptical in cross section),
as shown in figures 3-26 and 3-27. By making the
piston egg-shaped, it is able to fit the cylinder better
throughout its operational temperature range. Cam-
ground pistons are machined so their diameter is
smaller and more parallel to the piston pin axis than it is
perpendicular to it. When the piston is cold, it is big
enough across the larger diameter to keep from rocking.
As it warms up, it expands across its smaller diameter at
a much higher rate than at its larger diameter. This tends
to make the piston round at operating temperature. The
walls of the skirt are cut away as much as possible to
reduce weight and to prevent excessive expansion
during engine operation. Virtually all pistons in
automotive applications are cam ground.
Figure 3-25.Controlling piston expansion.