Crankshaft Keyway
A slot machined into the nose of the crankshaft designed to hold a key that aligns and locks the timing gear or harmonic balancer.
A crankshaft keyway is a precision-machined slot or groove cut into the snout (nose) of the engine's crankshaft. Its primary purpose is to accept a matching metal key (most commonly a Woodruff key), which acts as a mechanical lock to align and drive accessories like the timing gear, timing belt sprocket, or harmonic balancer. By physically locking these components to the crankshaft, the keyway ensures they spin at the exact same speed and angle as the engine crank.
This alignment is critical for engine timing; in interference engines, even a tiny shift in sprocket alignment can cause the pistons to strike the valves, resulting in catastrophic engine failure. The keyway must be machined to extremely tight tolerances to prevent play or backlash. Over time, if the crankshaft sprocket bolt or harmonic balancer bolt becomes loose, the key can rock inside the slot, wearing out (wallowing) the keyway and destroying the crankshaft timing alignment.
During an engine rebuild or timing chain service, mechanics must inspect the crankshaft keyway for signs of wear, rounding, or cracking. A damaged keyway cannot be easily repaired and usually requires replacing the entire crankshaft or sending it to a machine shop for custom welding and re-milling. Using a new key and ensuring the retaining bolt is torqued to spec prevents keyway failure.