SI class 12.9 high strength steel bolts were used to investigate the fatigue behavior of bolt threads rolled before/after heat treatment using five different preload values. Bolts were 3/8 UNRC-16 (coarse) and preloads were taken as 1, 50, 75, 90, and 100 % of roll before heat treatment proof stress. Since proof stress was lowered 10 % for roll after heat treatment, these preloads for roll after heat treatment bolts were then actually 1.1, 55, 83, 100, and 110 %. The tests produced a range of R-ratios between 0.03 and 0.92. Maximum near surface residual compressive stresses, obtained via X-ray diffraction, ranged from -600 to -1000 MPa. Axial loads were applied through the nut and all fatigue failures occurred at the first thread of the nut/bolt interface with crescent-shaped cracks dominating in most tests. SEM evaluation indicated all fatigue crack growth regions contained multiple fatigue facets, while final fracture regions contained ductile dimples. Cyclic creep/ratcheting was monitored and little, or none, was observed for preload tests equal to or less than 75 %. The fatigue resistance of the threads rolled after heat treatment with preload stresses of 1 % caused a 147 % increase in 10⁷ cycles fatigue strength compared to roll before heat treatment. This is in agreement with other roll before/after low R-ratio results. However, with higher preload stresses, the roll after heat treatment fatigue strength did not increase. Constant life Haigh diagrams at 10⁵ and 10⁷ cycles were not always in agreement with VDI 2230 bolt preload guidelines.