This parallel key transmits torque from the drive sprocket to the reel shaft within the header drive system. It fits into a machined slot in the shaft and the mating hub, preventing rotational slip under load. A worn or sheared key causes the reel to lose synchronization with the cutterbar, leading to uneven crop feeding. The original manufacturer’s key is cut to the exact width and height of the slot, so no shimming or filing is needed for a tight fit. On reassembly, ensure the key is fully seated before tightening the hub retaining hardware.