A surgical instrument having a transmission assembly, a transducer, and a handle assembly is configured for coupling the transmission assembly to the handle assembly using a locking mechanism. The locking mechanism is operable to restrict the rotational movement of an actuator coupling member and the transducer relative to the handle assembly. The locking mechanism may also be operable to lock a trigger of the handle assembly in a first position. An inner tubular actuating member of the transmission assembly may be threadably attachable to the actuator coupling member or the actuator coupling member may include a latching mechanism to couple to a flared portion of the inner tubular actuating member. A waveguide of the transmission assembly may also be threadably attachable to the transducer. In one alternative, the trigger may be configured to operate the locking mechanism, either in the first position or when pivoted distally to a third position.