Ornamental doorknobs on old mortise-style door locks loose their allure when the handle becomes loose or falls off.
Typically the problem can be solved by simply tightening the small handle screw located near the base of the knob. If the screw is lost or too worn to function properly, replacing it will usually solve the problem.
Unfortunately, the thread size on original handle screws is now obsolete. With a little luck, a No. 8 or a No. 10 oval head machine screw or a similar-sized set screw can be made to work.
Step 1: If the handle screw is still in place on the problematic knob, loosen the screw until the knob turns freely on the spindle.
Step 2: Hold the opposing doorknob on the other side of the door with one hand and turn the problematic knob counter clockwise to unscrew it from the spindle.
Step 3: Once the knob has been removed, pull the remaining knob and spindle out from the door.
Step 4: Take the knobs and spindle to your local hardware store to find the best replacement screw option. If an exact match cannot be located, choose a screw that fits into the knob hole but does not quite thread into the spindle. An 8-32 or 10-24 oval head screw will usually do the trick.
Step 5: Once a screw has been chosen, use a thread tap and handle or a thread-cutting screw to retap the spindle threads to match the new screw thread size. Insert the cutting end of the tap into the spindle hole and turn the tap slowly, clockwise, down into the hole. It is best to tap the hole all the way through the spindle, and then carefully back the tap back out of the hole, leaving the new threads intact.
A less expensive option for cutting new threads into the spindle is to use a thread cutting screw and turn it down and through the spindle instead.
Step 6: Once the threads have been cut, insert the opposing knob, with the spindle attached, into the door and thread the problematic knob back onto the open spindle end. Align the handle hole with the newly tapped spindle hole, insert the new screw and hand tighten it until the knob is secure and the door handle set is functional once again.