A drill string rod (1) to form part of an assembly (10) of connected such drill string rods (1, 1b), the drill string rod (1) comprising: an elongate central rod portion (2) extending axially between a male end (3) and a female end (4), wherein the central rod portion (2) is hollow-cylindrical defined by an inner first diameter (drod) and an outer second diameter (Drod), wherein the male end (3) comprises a spigot (5),wherein the spigot (5) comprises a base (6) projecting axially from a shoulder (7) that axially separates the spigot (5) and the central rod portion (2), wherein the female end (4) comprises a sleeve portion (8) configured to fit to the spigot (5), wherein the base (6) is provided with an outer thread and wherein the sleeve portion (8) is provided with an inner thread, wherein the inner thread corresponds to the outer thread such that the inner thread of the sleeve portion (8) is attachable to the outer thread of the base (6) of the spigot (5) of a further drill string rod (1b) of the assembly (10), wherein, in a radial plane (P) to the longitudinal axis (9) of the drill string rod (1), the base (6) of the spigot (5) is defined by an outer third diameter (Dspigot) and an inner fourth diameter (dspigot) and the sleeve portion (8) is defined by an outer fifth diameter (Dsleeve) and an inner sixth diameter (dsleeve), wherein the second diameter (Drod) is between 30 and 60 mm, and wherein the six diameters mentioned are selected within specific constraints.
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