A pipette tip mounting shaft includes outwardly circumferentially extending locking lobes over which the pipette tip collar is mounted. The locking lobes preferably include an inclining ramp portion that gently flexes and distorts the pipette tip collar out-of-round as the mounting shaft is inserted into the pipette tip, rather than the stretching tip collar. Each locking lobe also includes a declining ramp portion which extends upward along the mounting shaft. The peak of the lobes is preferably curved. When the pipette tip is fully mounted on the mounting shaft, a locking ring on the inside surface of the tip collar engages the declining ramp of the lobes to provide an over-center engagement, however, the required ejection force is small.