I've seen several phenolic timing gears with teeth broken off. That stops the engine in its tracks, and if an intake valve happens to be fully open when the cam stops, there can be piston to valve contact. I don't recall ever seeing an aluminum gear lose a tooth, and finding one with enough tooth wear to need replacement is pretty rare. There's also a steel cam gear that's made for Mercruiser marine engines that appears to be a direct swap for a stovebolt cam gear. I've got one in the shop that I intend to do a comparison with an aluminum gear to see if all the dimensions are the same.
Jerry
