Abstract: In a tachometer assembly for attachment to an output shaft of a rotary machine, wherein the tachometer assembly includes a tachometer housing enclosing a sensor drum fixed to a tachometer shaft, the shaft extending from the tachometer housing and adapted for attachment to the output shaft of the rotary machine for rotation therewith, and a sensor pad located peripherally adjacent the sensor drum, an improvement which includes an internal wrench mechanism for locking the tachometer shaft to the tachometer housing for common rotation therewith so as to facilitate threaded engagement between the tachometer shaft and the output shaft, as well as for unlocking the tachometer shaft from the tachometer housing for rotation relative to the tachometer housing to thereby permit the tachometer shaft to rotate with the output shaft after the tachometer housing is secured to the rotary machine housing.