I HAve a few basic questions concerning how to piece an exhaust system together:
1) How are exhaust systems pieced together? Specifically, how do you know/determine if one pipe/piece---i.e., muffler, converter, or pipe will fit over another?
For example in this picture, i am trying to fit a muffler or possibly an extension pipe over this opening which i've measured to be 2.5 inches. How do i know if the muffler will fit over this pipe? Or inside the pipe? What if the muffler opening is exactly the same as the pipe?

2) What does "I.D." mean?
What does "O.D." mean?