Andy, that is the blade boss. It is firmly pushed onto a taper on the crankshaft, and it takes a special trick to remove it. The usual way of applying the special trick is to rest the boss on a heavy piece of metal like an anvil, and hit the opposite point on the boss hard with a heavy hammer. Anvil and hammer-face both have to be parallel to the surface of the boss. It may take several hits. There may be an advantage in rotating the boss a bit between successive hits. Remember, the idea is to deliver an impact, to shift the inner surface of the boss momentarily so it can spring off the taper, so a very few, hard heavy hits is way better than playing pat-a-cake with it.
