|UNJF threads. "J" threads are made in both external and internal forms. The external thread has a much larger root radius than the corresponding UNC, UNR, UNK, or UNF threads. This radius is mandatory and its inspection is required, whereas no root radius is required on UNC, UNF, or UNEF threads. Since the larger root radius increases the minor diameter, a UNJF or UNJC fastener has a larger net tensile area than a corresponding UNF or UNC fastener. This root radius also gives a smaller stress concentration factor in the threaded section. Therefore, high-strength bolts usually have "J" threads.