A – The A stands for an “A” section v-belt. This is considered a “Classic” v-belt style that has a top width of ½ (0.5) inches and a thickness of 5/16 (0.312) inches.
78– 78 inches is the inside circumference of the belt. For A-section v-belts, the outside circumference is found by adding 2 inches to the inside length of the belt. So, the outside circumference of the A78 belt is 80 inches (78+2).