In present work, the friction stir welding (FSW) tools are designed with different surface areas of pin side. The transient temperatures during FSW were measured on advancing side (AS) of top surface, midpoint of the sample thickness, and center on the bottom surface. The results show that the appropriate ratio of pin side area for a defect free FSW of 7075-O Al alloy can be within the range of 23.79–29.83% at 500 rpm rotation speed and 50 mm/min welding speed. It was also found that a higher ratio of pin side area results in a joint with higher tensile strength. Using a semi-spherical pin with high ratio of pin side area (29.83%) produces a friction stir welded joint with higher tensile strength and finer grain size. Based on the presented results, it could be stated that the pin area ratio is one of the main technical parameters of friction stir welding.