What happened
The pilot reported completing a normal engine runup prior to initiating the takeoff roll. As the aircraft accelerated through approximately 75 knots, a loud bang was heard originating from the left side of the airframe. The pilot immediately aborted the takeoff and completed a safe rollout on the runway.
The investigation
Post-flight examination of the left engine compartment revealed that one of the three counterweights on the left propeller assembly was missing. Each counterweight is secured by a single bolt and locking nut. Inspection of the bolt tube showed evidence of pre-load stress, indicating the fastener had failed due to excessive tension.
Findings
A service bulletin issued on March 3, 1982, addressed this specific failure mode. The bulletin noted that reducing the torque specification from 105-100 foot-pounds to 65-60 foot-pounds would help reduce pre-load stress on the counterweight bolt. This maintenance error led directly to the in-flight separation of the component.
Propeller counterweight failure was the primary mechanical issue identified during the investigation.