An ultrasonic surgical instrument that includes an ultrasonic transducer, a handle assembly supporting the ultrasonic transducer, a clamp arm assembly, and a mechanical lockout assembly. The handle assembly includes a housing and an ultrasonic blade acoustically coupled with the ultrasonic transducer. The clamp arm assembly includes a clamp arm. The mechanical lockout assembly is configured to switch between at least an unlocked configuration and a locked configuration. In the locked configuration, the handle assembly and the clamp arm assembly are not completely coupled together and the operator is physically prevented from activating the instrument using an operator input feature. In the unlocked configuration, the clamp arm assembly and the shaft assembly are completely coupled together and the operator is able to activate the instrument using the operator input feature.
