1. Drilled a  3/4" hole through the center of the blank. Attached blank to lathe using a 3/4" bolt held in the chuck.

        2. Formed the back side of the back plate using measurements from the original 4" back plate.

        3. Removed the 3/4" bolt and turned the back plate around. Attach the back plate using the shoulder that was cut
            in pic # 2.  Bore the ID of the back plate to match the OD of the spindle thread root. Be careful and remember
            this is the ID where the threads will be formed.  For mine the hole was bored to 1.316"

        4. Created a shoulder wide enough for the chuck jaws to clamp to so the back plate can be turned around.

        5. Turn the back plate around and clamp to the shoulder created in pic # 4.  Measure closely and bore
            the ID of the back pate to allow for the shoulder of the spindle to fit correctly.  The remaining material
            will allow for the threads to be formed.

        After pic # 5   I cut the 4mm ID threads from the backside of the back plate. In other words the threading tool
        will enter from the back of the back plate. This is where the spindle test piece is needed.  Keep cutting threads
        until the test piece fits. Once the threads are complete the original chuck can be removed from the lathe and the
        newly completed back plate can be attached to the spindle. Once the new back plate is attached the face of
        the plate can be cut to match the chuck. The OD of the back plate can also be cut to match the OD of the new chuck.
        And last but not least you will need to drill the mounting holes through the back plate , I used a dividing head to
        space the holes evenly you can however do some creative measuring and make it work.