Camshaft key

70-1558 is the small Woodruff key that locates the camshaft pinion on the shaft so the gear can't spin loose against it. It's a simple, easy-to-overlook part, but if it shears or goes missing during a rebuild the timing gear won't stay put and the cam timing goes with it. You'll find it referenced in the crankshaft, camshaft, and timing gear sections of the parts books, which tells you it lives down in the timing chest area with the other gear-train hardware. Older parts books list this one under the E1558 number, so if you're working from an early catalog that's the same part. Given how small these keys are, it's worth having a spare or two on the bench any time you're pulling the timing cover for cam or gear work.