To maximize the lifespan and performance of your headset The crown piece must be carefully fitted to the seat for an accurate fit with minimal tolerances. The Park Tool CRC-1 Crown Race Cutting Tool does just that but it requires an adapter for 1 diameter tapered head tubes 5 inches. This is the adapter. It includes a precision machined adapter housing , as well as a milling cutter with a high-speed steel crown. Please note that this only works on forks which have a maximum tapered head tube length of 4.25 inches.