The effect of thread pitch (coarse and fine threads) on fatigue life of ISO bolts within the range of 10 D 24 mm (D ¼ outside diameter) and American Unified bolts within the range of 7=1600 D 100 was studied. The experimental results showed that ISO standard coarse threaded bolts have a higher fatigue life than the fine threaded bolts. For unified bolts, however, when the comparison is made on the basis of core diameter, coarse threaded bolts are superior to the fine ones, whereas on a nominal based comparison, both coarse and fine threaded bolts exhibit the same loading capacity. The results also indicated that for the same core stress, the fatigue life of both ISO and unified bolts, except for 100 unified bolts, decreases as the nominal diameter increases within the range tested. For 100 unified bolts, the fatigue life increases with the increase of nominal diameter.
