A dry sprinkler comprising: an outer structural assembly having a proximal inlet , a distal outlet , and an internal passageway extending between the inlet and the outlet defining a longitudinal axis (A) of the sprinkler , the outer structural assembly including: an inlet fitting including a proximal head portion and a distal body portion , the head portion having an external thread defining a nominal external thread diameter, the body portion including an external groove defining a nominal groove diameter being greater than the nominal external thread diameter, the external thread and groove providing the sprinkler with alternate threaded and grooved coupling arrangements for connection to a fluid supply pipe (BL), the inlet fitting having an internal surface with four sections (I,II,III,IV), each of the four sections (I,II,III,IV) defining a different portion of the internal passageway , the first section (I) defining an inlet portion with a sealing surface of the dry sprinkler , the second section (II) defining an expanding region of the internal passageway to a third section (III) that defines the widest portion of the internal passageway , and a fourth section (IV) that converges the internal passageway in an axial-to-distal direction, the inlet fitting having an outer surface portion that forms a transition between the proximal head portion and the distal body portion to define a transition portion between the external thread and the external groove , the transition portion defining a stop surface ; an outlet frame including a deflector axially spaced at a fixed distance from the outlet ; and a casing tube disposed between the inlet fitting and an outlet frame ; a seal assembly disposed along the passageway for contacting the sealing surface ; and a thermal trigger for supporting the seal assembly against the sealing surface in an unactuated state of the dry sprinkler . an inlet fitting including a proximal head portion and a distal body portion , the head portion having an external thread defining a nominal external thread diameter, the body portion including an external groove defining a nominal groove diameter being greater than the nominal external thread diameter, the external thread and groove providing the sprinkler with alternate threaded and grooved coupling arrangements for connection to a fluid supply pipe (BL), the inlet fitting having an internal surface with four sections (I,II,III,IV), each of the four sections (I,II,III,IV) defining a different portion of the internal passageway , the first section (I) defining an inlet portion with a sealing surface of the dry sprinkler , the second section (II) defining an expanding region of the internal passageway to a third section (III) that defines the widest portion of the internal passageway , and a fourth section (IV) that converges the internal passageway in an axial-to-distal direction, the inlet fitting having an outer surface portion that forms a transition between the proximal head portion and the distal body portion to define a transition portion between the external thread and the external groove , the transition portion defining a stop surface ; an outlet frame including a deflector axially spaced at a fixed distance from the outlet ; and a casing tube disposed between the inlet fitting and an outlet frame ; a seal assembly disposed along the passageway for contacting the sealing surface ; and a thermal trigger for supporting the seal assembly against the sealing surface in an unactuated state of the dry sprinkler . <EOS>