Final answer:
The most common type of cam used in magneto systems for small opposed engines is the lobe cam or high point cam, crucial for controlling ignition timing.
Step-by-step explanation:
The type of cam most commonly used in a magneto for a small opposed engine is typically a lobe cam or a high point cam. Magnetos are a key component in the ignition systems of engines and are specifically designed to generate a spark for igniting fuel in the engine's cylinders. In the case of an opposed engine, which often refers to engines with cylinders opposite each other, the cam serves as a critical part to control the timing of the spark generated by the magneto.