The invention relates to an assembly (1) for forming a mast for guiding a drill drive in an HDD machine. The assembly comprises a mast body (10) with a generally U-shaped cross-section, a mast cover with a recessed surface (21), a set of two or more racks (30), first connecting means (24) for connecting the racks releasably to the mast cover and second connecting means (44) for connecting the racks releasably to the mast body. The racks together with the mast body and the mast cover in assembled state form a mast with a substantially closed contour.