LO 7-2 Classify pistons according to their application, design, construction features, and material

1.4. Cam Ground Pistons

Cam Ground Pistons

A piston will expand when heated. How much it expands depends on the temperature and mass of the area that is heated. Hotter and thicker areas of a piston will expand more than thinner, cooler dimensions. If a piston’s operating temperature is excessive, or operating clearances between the piston and the cylinder wall are too small, the piston will begin to scuff and score and eventually could seize in the cylinder Figure 8. A number of construction features are used to help minimize the problem of piston expansion. The most common technique for piston construction is to use a shape that is carefully machined to allow for a controlled expansion of selected portions of the piston in the cylinder. The piston shape that is almost universally used to enable expansion without seizing is an elliptical or oval shape. A cam ground piston is made with the smaller diameter measured across the piston pin, giving it a slightly oval shape. When the piston heats up to operating temperature, it will then be round Figure 9. This piston configuration is commonly known as a cam ground piston due to its egg or elliptical shape.

FIGURE 07-8: A heat-resistant, graphite-based antiscuff coating is applied to the piston skirt to reduce friction and blowby and to create a tighter fit between irregularly shaped cylinder walls.

FIGURE 07-9: To permit piston expansion while reducing the looseness of the piston in the cylinder, pistons are machined elliptically and become round when warmed up.

Most piston expansion takes place in the dimensions where there is the most metal and heat. This means the crown will expand the greatest amount when heated. Because the wrist pin bore is reinforced, it will also expand the most. Areas that expand the least are the bottom of the skirt and the thinner skirt areas located at about 90 degrees from the wrist pin. This area corresponds to the major and minor thrust surfaces of the piston. Knowing this helps explain why measuring the piston diameter should be done at 90 degrees opposite to the piston pin bore; it is the widest cross section of the piston.

Aluminum pistons also possess a barrel or taper shape. The crown and ring belt parts of the piston are significantly smaller than the bottom circumference. A slight bulge in the middle section to reduce piston-to-cylinder clearance is built in to minimize noise and piston slap (piston contact with the cylinder wall) and to enhance sealing capabilities. Because the cam ground piston is irregularly shaped, rings (not the piston) are responsible for sealing most of the clearance between the cylinder walls when the engine is operated cold. This explains why piston rings on aluminum pistons will generally be thicker and require greater ring tension to ensure a gas-tight seal.