51-23-04\tSIKORSKY: Applies to All Model S-51 Helicopters. \n\nCompliance required as indicated. \n\n1.\tCompliance required prior to next flight. \n\nInspect all main rotor link assemblies (P/N S510348) received from the manufacturer between June 1 and September 6, inclusive for location of the identification stamp "F". Links with the "F" metal-stamped in the critical area on the inner faces of the ears should be retired from service. Those with the "F" stamped on the outer faces or on noncritical areas of the inner faces of the ears should be polished locally with emery and crocus cloth to remove the "F" and the links may be returned to service. (See Figure 6.) \n\n\n\nAD 51-23-04 \n\n(Sikorsky Information Circular No. 175, dated September 7, 1951, covers this same subject.) \n\n2.\tMain rotor link assemblies (P/N S510348) shall be retired from service when a total flight time of 960 hours on the links has been accumulated. \n\n(Sikorsky Information Circular No. 122, Revision B, dated September 10, 1951, covers this same subject.)