The previous study revealed that the new double thread bolts composed of a coarse single thread and coarse multiple threads (called DTB-II) have poor thread rolling formability and insufficient tensile strength. In this study, we developed two modified types of 3-thread DTB-II called 3-1 thread DTB-IIB and DTB-IIC: one thread was removed from the triple threads for both DTBs and the height of the multiple thread root-diameter was raised by half in the latter case at the point where a single thread was traversed by the bottom of multiple threads. The FEM thread rolling simulations were first performed using dedicated dies having a groove that followed the same outline as the thread profile of the modified DTBs. Both modified DTBs were rolled precisely and the thread heights reached the target level at any cross section. The actual forming states in the thread rolling experiments well reproduced the FE-simulation results. The performance evaluation tests demonstrated that the 3-1 thread DTB-IIC, even with a single-nut structure, has a practically sufficient tensile strength, and the fatigue strength and the antiloosening performance of both DTBs clearly exceed that of the conventional DTB-I composed of a coarse single thread and a fine single thread.
