Drill string rod with guidance shoulder

A drill rod assembly to form part of a drill string, the assembly comprising:

• a drill rod including a hollow elongate main length section extending axially between a male end and a female end, the male end including a spigot having a threaded section projecting axially from a shoulder that axially separates the spigot and the main length section, the shoulder having an outside diameter that is greater than an outside diameter of the main length section, wherein an axial length of the shoulder over which the greater outside diameter of the shoulder extends is greater than the outside diameter of the shoulder;
• and
• a drill bit releasably attachable to the male end of the drill rod;
• the drill bit having an axially forward cutting head and an axially rearward mounting sleeve to attach to the male end of the drill rod, wherein an outside diameter of the mounting sleeve is equal to or approximately equal to the outside diameter of the shoulder.