A biostimulator, such as a leadless cardiac pacemaker, including a fixation element that can be locked to a helix mount, is described. The fixation element includes a fastener that engages a keeper of the helix mount. When engaged with the keeper, the fastener locks the fixation element to the helix mount. Accordingly, the fixation element does not move relative to the helix mount when the biostimulator is delivered into a target tissue. Other embodiments are also described and claimed.
