The present invention provides a power unit (1) for a drug delivery device as well as methods for providing and mounting the power unit (1) in the drug delivery device. The power unit (1) comprises a first interface member (30) extending along a reference axis and comprising a hollow body (31) and a first engagement structure (35), a second interface member (10) comprising a tubular structure (11) extending at least partially through the hollow body (31) and having a skirt (12), and a second engagement structure (14) configured for releasable engagement with the first engagement structure (35), and a torsion spring (20) comprising a first end portion (23) being attached to the first interface member (30) and a second end portion (22) being attached to the second interface member (10), the torsion spring (20) being pre-strained to induce a relative rotational motion between the first interface member (30) and the second interface member (10). The first interface member (30) and the second interface member (10) are configured for relative motion along the reference axis from a first relative axial position in which the first engagement structure (35) and the second engagement structure (14) are engaged to prevent the relative rotational motion between the first interface member (30) and the second interface member (10) to a second relative axial position in which the first engagement structure (35) and the second engagement structure (14) are disengaged. In the first relative axial position of the first interface member (30) and the second interface member (10) the torsion spring (20) is accommodated in a user inaccessible space (39) defined by the hollow body (31) and the skirt (12).