The action of the main Crankshaft, slave rods and pistons appears complex at first sight, but is much more easily to understand when reversing the motion
of the engine e.g. having the crankshaft rotating instead of the engine for visibility purposes.
The gear ratio for the Main drive Gear and Magneto gears is two and a quarter. The contact breakers give 2 sparks per revolution and run at two and a quarter times the engine speed,
each giving nine sparks every two revolutions of the engine. The firing order therefore is 1, 3, 5, 7, 0, 2, 4, 6, 8.