A rotary cutting tool having a longitudinal axis and extending in a forward (D F) to rearward direction (D R), comprising: a tool holder having a holder longitudinal axis and a replaceable cutting head threadingly engaged to the tool holder, wherein: the replaceable cutting head has a head longitudinal axis extending in a forward (D F) to rearward direction (D R), the replaceable cutting head comprising: a forward portion forming a cutting portion; and a rearward portion forming a mounting portion, the mounting portion comprising a male coupling member having an external thread and protruding rearwardly from a head base surface, the head base surface extending transversely with respect to the head longitudinal axis, and defining a boundary between the cutting portion and the mounting portion, wherein: the external thread comprises an external thread ridge extending helically about an external thread axis and comprising forward and rearward external flank surfaces (46, 48) and an external top surface extending therebetween; the forward and rearward external flank surfaces (46, 48) generally face in opposite axial directions (D F, D R) and delimit a helical external thread groove that comprises an external bottom surface; the external thread of the male coupling member is a back-tapered conical thread having at least one thread diameter (d 1, d 2) which increases in the rearward direction (D R); the tool holder has a female coupling member having an internal thread extending rearwardly from a holder forward surface, the holder forward surface extending transversely with respect to the holder longitudinal axis; and the rotary cutting tool is adjustable between: a released position in which the internal and external threads (42, 72) are not threadingly engaged to one another, and a locked position in which the male coupling member is removably retained in the female coupling member with the internal and external threads (42, 72) threadingly engaged to one another; characterized in that: the internal thread of the female coupling member is a straight thread. a forward portion forming a cutting portion; and a rearward portion forming a mounting portion, the mounting portion comprising a male coupling member having an external thread and protruding rearwardly from a head base surface, the head base surface extending transversely with respect to the head longitudinal axis, and defining a boundary between the cutting portion and the mounting portion, wherein: the external thread comprises an external thread ridge extending helically about an external thread axis and comprising forward and rearward external flank surfaces (46, 48) and an external top surface extending therebetween; the forward and rearward external flank surfaces (46, 48) generally face in opposite axial directions (D F, D R) and delimit a helical external thread groove that comprises an external bottom surface; the external thread of the male coupling member is a back-tapered conical thread having at least one thread diameter (d 1, d 2) which increases in the rearward direction (D R); the external thread comprises an external thread ridge extending helically about an external thread axis and comprising forward and rearward external flank surfaces (46, 48) and an external top surface extending therebetween; the forward and rearward external flank surfaces (46, 48) generally face in opposite axial directions (D F, D R) and delimit a helical external thread groove that comprises an external bottom surface; the external thread of the male coupling member is a back-tapered conical thread having at least one thread diameter (d 1, d 2) which increases in the rearward direction (D R); the tool holder has a female coupling member having an internal thread extending rearwardly from a holder forward surface, the holder forward surface extending transversely with respect to the holder longitudinal axis; and the rotary cutting tool is adjustable between: a released position in which the internal and external threads (42, 72) are not threadingly engaged to one another, and a locked position in which the male coupling member is removably retained in the female coupling member with the internal and external threads (42, 72) threadingly engaged to one another; characterized in that: the internal thread of the female coupling member is a straight thread. a released position in which the internal and external threads (42, 72) are not threadingly engaged to one another, and a locked position in which the male coupling member is removably retained in the female coupling member with the internal and external threads (42, 72) threadingly engaged to one another; characterized in that: the internal thread of the female coupling member is a straight thread.