Why such large loads? Think about it: Neither shaft rotates smoothly, yet the gears force them to rotate together. The crank is accelerated four times per revolution by a 10,000-pound shove from a cylinder firing and slows between firings. This produces violent irregular, not smooth, motion. The cams experience alternately the torques required to lift and accelerate valves against their spring forces and inertia, followed by similar but reversed forces as the valves close. As this engine revved to 10,500, there were surely torsional oscillations excited in its crankshaft, and long, slender camshafts aren’t rigid either. Somehow, all this was adding up to forces that, from time to time, broke a gear tooth.