bit holder face 45 and base member face 46 and with the bit holder portion 39 via the leg ends 48b there is nothing to prevent the spacer 48 from rotating and this could cause undue wear additionally fines and the like may accumulate between the leg ends 48b and the open overhang of the bit holder face 45 in the region of such ends furthermore there is less bearing area for the underface 45 of the bit holder head 41 these possible shortcomings however may be overcome in various ways as will now be discussed in the various figures like reference numerals will be used to designate like parts thus referring now to fig1 9 a base member 35 is again illustrated as having coaxial perforations 37 and 38 designed to receive the portions 39 and 40 of a bit holder 36 the retainer to be received within the annular groove 42 is shown in this instance as comprising a split ring 43 fig6 this retainer 43 will eventually be received within the annular groove 44 provided in the base member 35 the spacer is also indicated in this instance as comprising a split ring 48 fig5 the arrangement of fig1 9 so far described is like that of fig1 14 but modifications have been made first the base member 35 has been provided with a boss 50 on its front face 46 adjacent the perforation 37 and having its major thickness being substantially the same as the thickness.