Front HS bulkheads enable diff height adjustment using eccentric bushings. Within the front side gearbox, the diff height has a 2mm adjustment range by which 1 Dot is for upper diff place. The diff height modification is a vital set-up ability that enables quick set-up adaption for either Carpet or Dirt songs. You can find 3 different angle configurations for adjustment for the front drive train. Carpet version runs with lower ride height also to keep consitently the drive shafts straight the differential is positioned in top position. Dirt version runs with upper ride height also to keep the drive shafts straight the differential is placed in lower place. Fits XB4'21