Abstract: A cat 40 tool holder wrench is provided. A wrench includes two knobs that protrude outward and perpendicular from a surface of the wrench at a top end of a u-shaped portion of the wrench. The knobs adapted to snuggly engage recessed fittings of a cat 40 tool holder to lock the cat 40 tool holder steady within a milling machine while a tool is changed from the cat 40 tool holder.