Abstract: An improved positive-grip chuck assembly, particularly suitable for use with high-speed air-driven dental handpieces, in which a tubular chuck has a radially-flexible bur-clamping section provided with an external shoulder, and the bore in which the chuck is received has an internal shoulder engagable with the external shoulder of the bur clamping section of the chuck to flex the bur-clamping section inwardly and to hold the same in a constricted condition for securely gripping the mid-portion of the shank of a dental bur. Piloting contact is made between the chuck and bur along circumferential zones spaced axially from the bur-clamping zone, and the rotor surrounds and braces the chuck portion defining the lower pilot zone to prevent distorting forces applied to the tip of the bur from being propagated to the bur-clamping zone.