51-22-01 SIKORSKY: Applies to All Model S-51 Helicopters.
Compliance required within the next 25 hours of operation, but not later than November 1, 1951.
Due to the reported failures of Gear Shaft (Generator Drive-Tail Drive Transmission System) P/N S-10-35-1004, all shafts in service shall be replaced by shafts which have been jig drilled (i.e., the taper pin hole is located squarely with respect to the splines) and which have been stoned and polished to deburr and remove any sharp radii where the taper pin hole intersects the splines.
(Sikorsky Service Information Circular No. 172, dated August 30, 1951, covers this same subject.)