The helicopter was on late final approach when the pilot attempted to open the throttle. The engine did not respond to the throttle movement and an autorotational descent was commenced. However, the rate of descent was not arrested and the aircraft landed heavily. An inspection of the aircraft found that the bolt attaching the throttle cable to the collective/throttle linkage was missing. The bolt and nut were later found on the floor of the aircraft between the seats. The helicopter had recently been stripped and repainted but during re-assembly a cotterpin was inadvertantly left out of the throttle control linkage. A subsequent periodic inspection failed to disclose the omission. The aircraft had completed appoximately 24 flight hours since that periodic inspection.
