[ursus]

Rotate the upper plate and the flex spring underneath will push it up and free of the groove it rides in. Ideally, the winged mount for the cam will be free and not stuck to the shaft. Be careful with the weight and spring assemblies and, if you dismantle it, be sure to replace each piece in its exact original position.

Also, one quirk of the Model-B distributor is that many - if not most - were made with an upper bushing that covers the drilling for the oiler, which defeats the purpose. If this is the case on yours, you might remove the oiler and carefully drill a hole through the bushing and clean any resulting burrs on the bearing surface.