After I had mended it I posted a request on another site asking how to do this. Thousands of views later I got no answer but peope asking how to do it ....So here it is
I just bought a D800 with large back focus issue ( about +40)for £260 (about $280) . It takes about 30 min to fix and re align. You need a small cross head screwdriver and a 1/16 or 1.5mm allen key. Take the battery out or you will regret it. Remove the rubber pad from the bottom ..it will go back fine. Stick it on a window so it does not pick up dust. Remove 8 screws holding the bottom.You can see where they are in the first photo below ,Not the 4 near the boss the other 4 (different lengths) and the four around and inside the battery door. The bottom plate will just lift off real easy. Ideal time to mask it up and respray with satin finish black.Beware the yellow battery retaining latch, if it comes off you will have a pig of a job to get it and the spring back. The second photo shows the chassis. Just to the right of the Allen key is where the chassis cracks..where it bends down just beyond the screw hole Mine was not cracked so the frame must have been bent. If the frame is cracked then stabilize the crack with some epoxy but turn the camera the right way up while it sets as if you get any dripping inside you is buggered. On the front edge of the camera you will see a cutout where the Allen key enters and also to the left in the circuit board. Down inside at 45 degrees are two shiny hex head screws. These are the adjusters for the focus module. Take a lens that you know the fine focus setting for ,preferably 70mm + .I used a 24-85 which I know needs +2 and I used it at 85mm . Set the backfocus in the menu to +2 or whatever your lens needs. If you are backfocusing turn each of the adjusters clockwise .I suggest two turns to start with. Then re fit the bottom or you cannot get the battery to stay in ..you only need a couple of screws to hold the base plate and check the back focus with an angled chart in the normal way. The focus point should now be on the scale .use small movements of the adjuster clock or anti clock to get it to zero. You can use any lens from any Nikon provided you know the fine focus adjust figure .the figure is the same when fitted to any camera. Job done refit the base plate and don't use the threaded boss again. 30 min if you don't loose the latch spring.