Revolutions per minute (rpm) is the speed at which the crankshaft rotates. Since the piston is connected to the shaft, the rpm, along with the length of the stroke, determine piston speed. Since, during each revolution, the piston completes one up-stroke and one down-stroke, piston speed is obtained by multiplying the rpm by twice the length of the stroke. This speed is usually ex-pressed in feet per minute (fpm). If the stroke is 10.5 inches (or 10.5/12 of a foot), and the speed of rotation is 720 rpm, the piston speed is computed as follows: