Is the shaft slot "horizontal"? There's a chance the last guy didn't have the pin and hole lined up and the pin is just smashed down on the surface of the shaft...can you turn the shaft with a big screwdriver?
Just a thought...
Mike B

Mike , the slot is vertical. I compared it to another SM420 I have and it looks correct. I have bumped the shaft and it does not move. Looks like some fun is about to happen.