The fractured end of a piston rod of a hydraulic press failed in line with the leading face of the piston retaining nut. Although the nut apparently had been seated uniformly, the face was polished, indicating that relative movement between it and the piston had taken place. Failure resulted from the culmination of two principal fatigue cracks which developed on approximately parallel planes from the roots of adjacent threads. A longitudinal section through the screw thread on the piston rod showed it had been carburized but not hardened, and that subsequent surface de-carburization to a depth of approximately 0.001...