thus operated in succession as the bias unit rotates each in the same rotational position so as to displace the bias unit laterally in a selected direction the selected rotational position of the shaft 21 in space thus determines the direction in which the bias unit is actually displaced and hence the direction in which the drill bit is steered fig3 and 4 show in greater detail the construction of the components of the disc valve 15 the disc valve comprises a lower disc 136 which is fixedly mounted for example by brazing or glueing on a fixed part of the body structure of the bias unit the lower disc 136 comprises an upper layer of polycrystalline diamond bonded to a thicker substrate of cemented tungsten carbide as best seen in fig4 the disc 136 is formed with three equally circumferentially spaced circular apertures 137 each of which registers with a respective passage 14 in the body structure of the bias unit the upper disc 138 is brazed or glued to a shaped element on the lower end of the shaft 21 and comprises a lower facing layer of polycrystalline diamond bonded to a thicker substrate of tungsten carbide as best seen in fig3 the disc 138 is formed with an arcuate aperture 139 extending through approximately 180 the arrangement is such that as the lower disc 136 rotates beneath the upper disc 138 which is held stationary with.