Noticeable free play in the piston knob is inherent in the design and not a sign of wear. On a 146 some 340 degrees, almost a full turn, is normal before the piston actually begins to move. On older models the dead travel is greater still; on a 149 from the 1950s it can run to several complete turns.
The reason lies in the construction: the first turns release the knob from the thread at the end of the barrel, and only then does the piston mechanism itself engage. The two movements are deliberately kept apart so that thread and piston are not driven at the same time and the difference in resistance stays perceptible. There is nothing to adjust here, as the mechanism offers no means of adjustment; a second-hand pen is therefore not worn out on this account. Cause for concern arises only when the piston becomes stiff, will not move at all, or ink leaks into the barrel – at that point the seal is due for renewal, which is a job for a workshop.