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.