A medical injector comprising: a set (12) pen needle (20) including: a hub (22) a needle (20) attached to said hub (22), said needle (20) having a distal end (19), formed for insertion into a patient and a proximal end (21) an injector body (14) having a tubular shape with a distal end (15) and a proximal end (17) and a reservoir (16) for medication sealed by a septum (18), said reservoir (16) enclosed in said injector body being wherein a guide wedge (24) is fixed to one of said injector body or said centerpiece (22), a track (26) defined being on the other of said injector body or said centerpiece ( 22), said track (26) a first portion (28) extending parallel to a longitudinal axis of said injector body (14) and a second part (30) arranged transversely to said first portion (28) and wherein, said central part (22) is formed to be telescopically mounted on said distal end (15) of said injector body (14), said guide key (24) received in said track (26) being, and wherein, in mounting said hub (22) of said assembly (12) pen needle (20) in said injector body, said first portion (28) of said track (26) is configured to receive said guide key (24) and for guiding said guide key (24) in a straight line along a distance sufficient to permit said proximal end (21) of said needle (20) fully pierces through said septum ( 18), without rotation of said needle (20), said proximal end (21) of said needle (20) guided to a location proximal of said partition (18), and said second portion (30) of said track (26) is configured to allow said assembly (12) pen needle (20) is rotated with said guide key (24), which is received in said second portion (30) of said track (26), said proximal end (21) of said needle (20) located proximal to said partition (18), characterized in that said track (26) further has as a third being part (38) arranged transversely to said second portion (30) of said track (26), and further comprises a spring (40) disposed between said injector body (14) and said
