Bill, it took me months to understand this once I had it figured out. I couldn't put it all together until I worked it out on a Single cylinder bike. It wasn't until then that the 720 degree thing made perfect sense. One cycle every two crank rotations.
Then twins were easy to figure after that. Of course, nothing is simple, because I was then hung up on how to count rpm's on a single because I didn't know about the wasted spark. Arhhgg!
