Integrated rear axle adjuster blocks make these a clean install and a solid mount for the slider spools Captive-style axle blocks makes chain adjustment a breeze, includes a long reach Allen wrench for easy first-time install Blocks are laser etched with chain alignment marks for easy alignment Blocks are available in anodized colors Replaceable spools are machined from UHMW-PE known for its excellent low drag and high wear compared to Delrin plastic of lesser brands, and mounted to the blocks with stainless hardware