904 interchange question

I sorta understand them. One thing I do understand is that most people are very lucky if they can just take it apart and get it back together correctly the first time and without reinstalling bad hard parts. It's a mechanical/hydraulic machine that's affected by heat and pressure that has nothing in common with a car's engine. To top it off, the first fifty that you disassemble will be different in what they need. There are no books or videos out there that will prepare you with everything that you need to know so that you don't make any mistakes.