The size of an
engine cylinder is indicated in terms of bore and stroke (fig. 2-24).
Bore is the inside diameter of the cylinder. Stroke is the distance
between top dead center (TDC) and bottom dead center (BDC). The bore is always mentioned
first. For example, a 3 1/2 by 4 cylinder means that the cylinder bore, or diameter, is 3
1/2 inches and the length of the stroke is 4 inches. These measurements are used to figure
DISPLACEMENT is the volume of space that the pistondisplaces, as it moves from one end
of the stroke to the other. Thus the piston displacement in a 3 1/2-inch by 4-inch
cylinder would be the area of a 3 1/2-inch circle multiplied by 4 (the length of the
stroke.) The area of a circle is pR2 , where R is the radius (one half of the diameter) of the
circle. With S being the length of the stroke, the formula for volume (V) is the
V = pR2 x S
formula is applied to figure 2-22, the piston displacement is computed as follows:
- R = 1/2 the diameter = 1/2 x 3.5 = 1.75 in.
- p = 3.14
- V = p (1 .75) 2 x 4
- V = 3.14 x
- V = 38.43 cu in.
The total displacement of an engine is found by
multiplying the volume of one cylinder by the total number of cylinders.
38.43 cu in. x 8
cylinders = 307.44 cu in.
displacement of the engine is expressed as 307 cubic inches in the English system. To
express the displacement of the engine in the metric system, convert cubic inches to cubic
centimeters. This is done by multiplying cubic inches by 16.39. It must be noted that
16.39 is constant.
307.44 cu in. x 16.39 =
cubic centimeters into liters, divide the cubic centimeters by 1,000. This is because 1
liter = 1,000 cc.
1,000 = 5.0389416
displacement of the engine is expressed as 5.0 liters in the metric system.