Lesson 1/Learning Event 1
Since the piston must be free to slide up and down inside the cylinder, there must be clearance between the
piston and cylinder wall. There must not be too much clearance though or the piston skirt will slap against the
wall. This would put excessive strain on the piston and prevent the piston rings from doing their job properly.
As metal warms up, it expands or gets bigger. Different kinds of metals expand different amounts for the
same temperature change. Pistons expand more than the cylinders. Therefore, this presents a problem in
keeping the piston to cylinder wall clearance proper as the engine temperature changes.
FIGURE 21. CAM-GROUND PISTON.
Builders of engines make pistons so their running clearances stay about the same regardless of temperature.
One common way of doing this is to grind the piston skirt so it is elliptical or egg-shaped instead of round.
When the engine is cold, the two high spots of the piston skirt fit close to the cylinder wall preventing piston
slap. As the temperature rises, expansion causes the piston to become round instead of binding up in the
cylinder. A piston shaped this way is called a cam-ground piston.
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