BBB BTL-125 Chain Checker Bicycle Cycle Bike Wear Guage Tool Silver. This is an easy-to-use chain wear checking tool. When the 0.75 = 75% wear mark is reached, the chain must be replaced soon. When the 1.0 = 100% wear mark is reached, the chain must be replaced immediately. It features an integrated chain hook that holds links during assembly. The added valve tool and 5/6/8mm hex wrench make this a unique multi-tool. Suitable for use on Road, MTB, Urban, Racing, and Mountain Bikes.