Designed to work only with Trek Knock Block system|The top cap on these headsets have unique raised features that key to Knock Block-compatible stems (made by Bontrager)|If spacers are needed you will need to use special Knock Block-compatible spacers with notch features|Lower headsets on bikes with Knock Block do not have any unique features as they are usually IS headsets that are available separately.|Special Knock Block-compatible headset spacers with notch features are required if you plan to use spacers.|Upper headsets ship with a star nut which can only be used on forks with steel or aluminum steerer tubes. For forks with carbon steerer tubes you will need a compression plug (sold separately). (Never use a star nut on a carbon steerer tube).|Precision Headset Knock Block Spacers › See more product details