So what if you pull your metal pipe off that back of your heads and the hole is not 5/8, what does that mean?

 

what are you trying to do?? delete the air injection??? if so you can pull the tube off but it'll probably be so carboned up the threads will be hard to access unless cleaned well...you can try and run the correct size bolt in or trick flow sells thermactor plugs...which is the system you are working on = thermactor air injection