Front HS bulkheads enable diff height adjustment making use of eccentric bushings. Inside front side gearbox, the diff height has a 2mm modification range which 2 Dot is for middle diff position. The diff height modification is a vital set-up ability which allows quick set-up adaption for either Carpet or Dirt songs. Carpet version operates with lower ride height and also to keep carefully the drive shafts directly the differential is put in upper place. Dirt version operates with top ride height and also to keep carefully the drive shafts straight the differential is put in lower position. Fits XB4'21