I found this site that does a good job of showing the differences in a 8" and 9" axle http://www.fordmuscle.com/blog/8-inch-or-9-inch/112192 This is a 9, can't get the socket on the bolt: This is an 8, can get the socket almost fully on: I'm going to look at what's supposed to be a 9" from a '71 mustang today and I wanted to make sure I wasn't buying something I already had