If it came off on its own, it's either worn out on the inside diameter or it's cracked. Inspect it closely for wear, and also measure the diameter of the crankshaft snout. Either way, you're probably going to need to pull the radiator. You can either drive it back on with a big hammer, or drill and tap the crankshaft for an installer tool. The factory used a special fixture to drive the balancers on, so people who predict all sorts of dreadful consequences for the main bearing thrust flange are talking through an "alternative orfice".
Jerry
