5.1 This section covers requirements of external threads with a mandatory thread root radius between 0.18042P and 0.15011P. Radius blends occur with the flanks of the thread at a maximum point equal to 75% of basic thread depth. The ANSI/ASME B1.21 or ISO 5855-1 MJ thread form shown in Fiqure 13 conforms to the M form shown in Figures 5-7 except for modification of the thread root.