A ultrasonic transducer assembly of an ultrasonic surgical instrument includes a horn, a piezoelectric stack positioned proximally of the horn and defining a longitudinal opening extending therethrough, a proximal end mass positioned proximally of the piezoelectric stack and defining a longitudinal opening, and a rod secured to the horn and extending proximally from the horn through the longitudinal opening of the piezoelectric stack and the longitudinal opening of the proximal end mass. The rod is secured to the proximal end mass to pre-compress the piezoelectric stack between the horn and the proximal end mass.