A helicopter main rotor bolt failed in the black-coated region between the threads and the taper section of the shank during assembly. The torque applied was approximately 100 N·m (900 in.·lbf) when the bolt sheared. No other bolts were reported to have failed. The failed bolt material conformed to AISI E4340 steel, as specified. The microstructure was tempered martensite, with hardness ranging from 41 to 45 HRC. Failure was in the shear ductile mode. The crack initiated in the area of slag inclusions. Inspection of other bolts from the same shipment was recommended.