Headset Conversion Kit 1 1/8" to 1" Reducer shims fit a standard frame head tube Standard frame head tubes are ID 34mm +/- 2mm or 1 5/16" Mainly used to put a fork with a 1" threaded steering tube onto a frame that is made to accept a fork with a 1 1/8" steering tube Pop out the cups in your frame head tube then install the kit