If you're lucky enough to have original hardware on the doors of your older home, eventually you'll find yourself needing to fix a wiggly doorknob. While a lot of modern door hardware is made from stamped metal parts with a limited life span, older items are sturdy enough to repair. On a mortised lock-set, the doorknobs are threaded onto a steel spindle and held in place with a setscrew. After years of wear, the screw threads finally give out. Rather than search salvage yards for matching knobs, do what your great-grandfather would have done and fix the ones you have.

1. Remove the old setscrew from the knob if it's still in place using a screwdriver with a small tip. If the screw spins but won't come out, push on the rear of the screw from inside the knob shank with a piece of stiff wire at the same time.
2. Take the knob and old setscrew to the hardware store for size comparison. Buy a new setscrew one size up in diameter from the old screw.
3. Pad the jaws of a bench vice with a soft cloth. Clamp the metal shank of the doorknob in the vice.
4. Consult a tap drill sizing chart to determine the proper drill bit to use for the oversize setscrew. As an example, to drill a hole prior to tapping threads for a setscrew with 8-36 National Fine thread, use a 9/64-inch drill bit.
5. Chuck the selected drill bit into a power drill. Drill out what remains of the threads in the metal shank of the doorknob. Hold the drill firmly and drill straight down through the setscrew hole.
6. Select a tap that matches the new setscrew threads. Lock the tap into a T-handle. Spray the setscrew hole and the end of the tap with penetrating oil. Insert the tap into the setscrew hole and twist the T-handle clockwise to start the threads. Cut threads by turning the T-handle one full turn clockwise, then one-half turn counterclockwise. Thread the entire length of the setscrew hole, then gently turn the tap counterclockwise to remove it.
7. Remove the doorknob from the vise. Hold the knob in the cloth with the spindle opening pointed downward. Spray oil into the setscrew hole to clean out any metal shavings. Wipe the knob off thoroughly with a clean cloth.
8. Install the new setscrew and check to ensure it moves freely in the freshly cut threads.
9. Thread the doorknob onto one end of the spindle. Insert the spindle through the door lock and thread on the other knob. Adjust the knobs on the spindle to achieve the desired clearance from the lock hub. Tighten both setscrews securely against the spindle for wiggle-free operation.

References

• This Old House: The Beauty of Glass Door Knobs
• AutoSpeed: Producing Threads Using Taps and Dies
• Lincoln Machine: Tap Drill Chart
• KMS Tools: Hand Taps & Proper Tapping Techniques

Resources

• CSG Network: Number Machine Screws Tap Drills Chart

Tips

• For a quick temporary repair of a doorknob with stripped setscrew threads, wrap a wisp of steel wool around the screw and thread it into the knob, then tighten securely against the spindle.