Help with John Deere 21" mowers.

I have been working on some john deere walk behind 21 inch mowers as a hobby for several years. I would like to learn more about some of the more difficult repairs. I don't know much about how the transmissions work. I am having difficulties with taking apart the BBC's and getting them to work right once back together. I am also not familiar with how to take out a crank shaft and replace it. I was hoping that some members of the forum are more experienced than I and they would be willing to share some of that information.

Thanks,

Doug