8 1/4 rearend

If your going to replace the open unit with the SG unit only, you can probably do it yourself with a little mechanical knowledge. Main thing you'll need is a dial indicator to find out what your gear backlash is before you take it apart so you can set up the same when you put it together. if your changing gears too thats a whole lot harder, you may want to let a knowledgable tech do it for you.