Friction drilling of a bore perpendicular to profile thickness facilitates the successive forming of internal threads with a high length and diameter larger than the profile thickness. Pre-study investigations declared that the creation of a pilot bore before manufacturing processes hinders the bursting and improves the surface quality of the expanded external profile’s wall by friction drilling and thread forming. However, the influence of the pre-drilling diameter on the mechanical properties of the manufactured threads has not yet been investigated. Therefore, this study evaluates the influence of the pre-drilling diameter on the fatigue behaviour of manufactured internal threads perpendicular to the thin-walled AZ91 profile thickness. To gain this aim load increase tests at room and elevated temperatures were performed on the manufactured M6 threads with pre-drilling diameters of 2.0 and 2.5 mm. Additionally, constant amplitude tests were conducted at room temperature in order to present the lifetime under various loads in a S-N (Woehler) diagram. It could be observed that the fatigue properties of the samples manufactured with a pre-drilling diameter of 2.0 mm improved slightly compared to the samples with a pre-drilling diameter of 2.5 mm. The manufactured M6 threads were mostly sheared off by fatigue tests in load direction (thread stripping) implying an internal thread failure caused by shear stresses due to incomplete formed thread profiles. This can be attributed to an unsynchronized rotation of the thread forming tools with tool’s holder or lack of available slip systems for cold forming of AZ91. The metallographic investigations declared a rhombus-shaped deformation region around the manufactured thread, which was created by severe plastic deformation during the friction drilling and thread forming process. Shearing the material near the subsurface of the formed thread profile indicated an enhanced hardness in comparison to as-cast material.