The invention relates to a pipette tip mounting shaft configuration and a disposable pipette tip having a matching configuration. The mounting shaft includes a locking section located above a lower sealing section. The locking section has outwardly extending locking lobes circumferentially spaced around the mounting shaft and located above a stop member that separates the lower sealing section from the upper locking section. When the mounting shaft is fully inserted into the collar of the mating disposable pipette tip, the tip locks onto the mounting shaft. The bore of the pipette tip includes a circumferential shelf separating its upper collar from the tip sealing area which is located below the circumferential shelf. The locking lobes gently flexes and distorts the pipette tip collar out of round as the mounting shaft is inserted in the pipette tip, rather than stretching the tip collar. The circumferential shelf on the pipette tip between the tip collar and the barrel isolates the sealing area at the upper end of the barrel from distortion, thus facilitating reliable sealing engagement.