B – The B stands for a “B” section v-belt. This is considered a “Classic” v-belt style that has a top width of 21/32 (0.656) inches, but is commonly referred to as a 5/8 (0.625) inch belt. The thickness of the belt is 13/32 (0.406) inches.
71– 71 inches is the inside circumference of the belt. For B-section v-belts, the outside circumference is found by adding 3 inches to the inside length of the belt. So, the outside circumference of the B71 belt is 74 inches (71+3).