An instrument (12) for assembling a bone anchoring device (11) is provided. The device includes a shank (2), which has a shaft section (21) to be anchored in a bone and a engagement end (22), a head (3) being separate from the shank (2) and having a through-hole (32) extending therethrough, and a receiving part (4) having a first end (41), a second end (42) and a longitudinal bore (48) extending from the first to the second end, the head (3) being positionable within that bore (48) at the second end. The instrument (12) comprises: a holding portion (9) arranged to hold a pin (8), which has a rod section (81) with a length and a diameter suited to fit and extend at least through the bore of the head (3), and which has a free end portion (82) extending from the rod section (81) such as to project from the through-hole (32) of the head when the pin is inserted, and configured to engage with the engagement end (22) of the shank. In one specific embodiment, the instrument further has a spring force means (10), wherein the pin (8) further comprises: a support section (85) that is adapted to move with respect to the holding portion (9) along a longitudinal axis of the pin (8) against a resilient force due to the spring force means (10).
