Wolf Tooth Headset Upper Knock Block - 28.6 8mm BlackWolf Tooth Precision Headsets for Trek Knock Block are machined in the USA out of aircraft grade aluminum with superior sealing for protection against the elements. Available in a variety of colors to add some style to your bike. These upper headsets are for Trek frames with molded in internal bearing cups. Designed to work with Trek's Knock Block system The top cap on these headsets have unique raised features that key to Knock Block compatible stems