This is a new creation from the guys at UFC, they have been testing out different internal bell diameters to suit the FS clutch shoe and they believe to have found improvements running a 53.30mm ID which extremely thick and thermally stable walls to the bell! Note: This bell is designed for Blackbone gear fitment only and for use with FS clutch shoes. The clutch bell has a 15mm shaft so you will also need to use either the Taylor or Turtle clutch bell carrier and backplate as well.