The present invention relates to a power unit for a medicament delivery device comprising a power body (24) having opposite proximal and distal ends, a plunger rod driver (44) coaxial and movably arranged in relation to said power body, a plunger rod (72) coaxial and movably arranged in relation to said plunger rod driver, a force spring element (78) operably connected between said plunger rod (72) and said power body wherein the plunger rod driver comprises first holding elements (66, 68) arranged to interact with second holding elements (76) on said plunger rod (72) and with the power body for releasibly holding said plunger rod (72) with said force spring element (78) in an injection tensioned state third holding elements (56) arranged to interact with fourth holding elements (41) on said power body (24) for releasibly holding said plunger rod driver in a penetration tensioned state first attachment elements (52) arranged to interact with second attachment elements (108) on a medicament container holder (92), capable of attaching the two attachment elements together and first blocking elements (62) arranged to interact with second blocking elements (43) on said power body (24), for blocking any movement of said plunger rod driver (44) in the distal direction after the plunger rod driver is released from a penetration tensioned state.La présente invention concerne une unité motrice destinée à un dispositif dadministration de médicament comprenant un corps moteur (24) possédant des extrémités proximale et distale en regard lune de lautre, un conducteur (44) de tige de piston coaxial et disposé mobile par rapport au dit corps moteur, une tige (72) de piston coaxiale et disposée mobile par rapport au dit conducteur de tige de piston, un élément (78) ressort imprimant une force relié fonctionnellement entre ladite tige (72) de piston et ledit corps moteur le conducteur de tige de piston comprenant de premiers éléments de maintien (66, 68) disposés de manière à interag
