Abstract: A mower blade locking joint for fastening and securing a rotary cutting blade to a vertically aligned spindle driven by an engine. The spindle extends below a mower deck. The blade has a pair of opposing cutting ends and a mounting hole between the cutting ends. The joint includes a threaded section, a lobed section, and a collar. At least one of the spindle and the mounting hole have seven lobes. The lobes have continuous radii, an inner or minor diameter, and an outer or major diameter. The outwardly directed lobes have the same dimensions as the inwardly directed lobes, and both the spindle and the blade have the same cross sectional or surface area. The inner or minor diameters of the spindle and mounting hole are at least 75% of the major or outer diameters.