Most of the old door knobs fit on a parcel threaded spindle that is held in place by a set screw. You can buy the spindles on line if you do not have the one that came with the knob. I put the knob on the spindle and measure the length of the remaining spindle and drill a hole in the top of the shank and epoxy the spindle in. If you do not epoxy the knob in place you can change out different knobs that fit the spinal.
