Abstract: A dental handpiece operable by compressed air having a turbine rotor, a drive sleeve for holding a dental instrument and coupled with the rotor by a ball-type planetary transmission, and an adjustment member operable on the planetary transmission so as to increase the torque transmissible by the planetary transmission. The transmission comprises inner and outer rings, with the balls frictionally engaged therebetween, the balls being in drive transmitting relationship with the rotor or the drive sleeve. A first of the rings is non-rotatable and a second of the rings is coupled with whichever of the rotor or drive sleeve is not associated with the balls. One of the rings is axially moveable relative to the other ring by the adjustment member in order to increase the radial load applied between the inner and outer rings through the intermediary of the balls whereby the torque transmitted by the planetary transmission can be increased.