The present invention relates to a power unit for a medicament delivery device comprising a power body having opposite proximal and distal ends, a plunger rod driver coaxial and movably arranged in relation to said power body, a plunger rod coaxial and movably arranged in relation to said plunger rod driver, a force spring element operably connected between said plunger rod and said power body wherein the plunger rod driver comprises first holding elements arranged to interact with second holding elements on said plunger rod and with the power body for releasibly holding said plunger rod with said force spring element in an injection tensioned state third holding elements arranged to interact with fourth holding elements on said power body for releasibly holding said plunger rod driver in a penetration tensioned state first attachment elements arranged to interact with second attachment elements on a medicament container holder, capable of attaching the two attachment elements together and first blocking elements arranged to interact with second blocking elements on said power body, for blocking any movement of said plunger rod driver in the distal direction after the plunger rod driver is released from a penetration tensioned state.
