An ultrasonic surgical instrument includes a reusable handle assembly and a removable and disposable shaft assembly. The handle assembly includes a trigger, a housing having a distal aperture formed in a distal end of the housing, and a drive member in communication with the trigger to actuate the drive member. The shaft assembly includes a proximal shaft portion, a rotator knob having a coupling feature, a transmission assembly extending distally from the proximal shaft portion, and an end effector coupled to the distal end of the transmission assembly. The drive member of the handle assembly is removably coupled to the proximal shaft portion of the shaft assembly. Another version includes a drive member of the handle assembly configured to removably engage a proximal shaft portion of the shaft assembly via a biasing member. Another version includes a waveguide of the transmission assembly non-threadably coupled to a transducer.