Six millimeters. According to Douban's inquiry. The special tool kit for orthopedic external fixator ⅸ mainly consists of 4 mm Allen wrench, 4× 55 screwdriver, 6× 80 screwdriver, 6 mm Allen wrench, adjustable wrench, thumbwheel, stable reset wrench, 4 mm screw T wrench, 6 mm screw T wrench, steel needle hand drill (optional), 5 pressure tractor (optional), 8 pressure tractor (optional) and instrument box.