The flywheel started life as a piece of 4" steel bar, held in the 4-jaw, and  the outer face was turned. The smaller diameter part that sticks out is what was used to start the engine in the good old days before electric starters, a length of strapping was wound around this and then given a sharp pull, much like a recoil start.

It was then parted off and the other face turned finishing off with a round nosed tool to give a fillet to the internal corner as you would find on a casting.

With a boring bar held upside down and cutting on the far side of the lathe the taper was cut.

Doing it that way allowed me to turn the matching taper on the crankshaft at the same top slide setting.

While the crank was in the lathe I also screwcut the M8 thread to take the flywheel retaining nut.

To stop the flywheel rotating on the taper it has a keyway for a Woodruff key. To cut this I made up a tapered guide bush and then pushed a 1/8" broach through

Then the matching slot for the woodruff key was milled into the crankshaft.

While I was at it, the short taper was cut on the other end of the crank for the timing cam (you can just see it on the right) and the taper in the cam cut at the same setting

To complete the cam a very shallow recess is milled part way round the circumference which is just enough to open and close the ignition contacts.

See Part one here  part two  part three  part four  part five  part six  part seven  part eight  part nine  part 10