Abstract: A timing sprocket is secured to a camshaft by a locking coupling so that before the coupling is tightened the sprocket will free wheel on the shaft for adjustment purposes and yet be firmly supported against tipping under load, and when tightened the sprocket is locked to the camshaft. The sprocket has a bore fitting on the shaft and a counterbore accommodating a longitudinally split locking ring. A taper between the bore and the counterbore matches a taper on the end of the locking ring. A bolt axially threaded into the camshaft end moves the locking ring from the loose condition where the sprocket can rotate on the shaft to a tight condition where the tapers compress the ring against the shaft and clamp the sprocket for rotation with the camshaft.