Why is my mortice lock stiff, and what do I do if it will not open?
Telling a worn key from a dry lock, a distorted case and a door bearing on its keep — and what can be done to a mortice lock without taking it out.
- Difficulty
- beginner
- Time
- 30 min
- Read
- 3 min
- Safety
- caution
Short answer
Test the key with the door open. If it turns easily open and stiffly closed, the bolt is bearing on the keep and the door or frame has moved. If it is stiff either way, oil the case through the forend slots and try the spare key. If it is still stiff, the case is worn or distorted and the lock needs replacing.
A mortice lock has very few things that can go wrong: the levers can wear, the case can be pinched by overtightened screws, the key can be worn, or the whole thing can be fighting a door that has dropped. The order below separates those in a few minutes without removing anything.
Safety
Step by step
- Test with the door open.Swing the door clear of the frame and turn the key. This removes the keep from the equation. Easy open and stiff closed means the door has moved and the lock is fine. Stiff both ways means the lock or the key.
- Try a different key.Lever keys wear at the bit, and a worn one has to be forced to lift the levers to their proper height. If a rarely used spare turns easily, cut a new key from the spare rather than from the worn one.
- Oil through the forend.There are slots in the forend around the latch and bolt. A few drops of light machine oil there, then work the key and the latch twenty times. This alone cures a great many stiff mortice locks, particularly ones that have not been used for years.
- Slacken the forend screws and try again.Overtightened screws, or screws driven into a mortice that is a fraction narrow, pinch the case and bind the mechanism. Back both off a turn and test. If it frees up, the recess or the mortice needs easing rather than the screws needing tightening.
- Check the keyhole is not fouling the key.Look through the keyhole from both sides. If the timber slot is tight against the key's shank the key is being deflected as it turns, which feels exactly like a stiff lock. Ease it with a narrow chisel and fit a larger escutcheon.
- Look at the keep if it is only stiff closed.Wipe the keep clean and look for a bright rub mark where the bolt is bearing. That mark tells you which way the door has moved. Adjust the door or refix the keep so the bolt enters the middle of the aperture.
- Check the door has not swollen.A timber door that has taken up moisture in a wet spell grows across its width. The lock then has to drag a swollen door into a frame. If the stiffness is seasonal, treat the door rather than the lock.
- Replace the lock if none of that works.Mortice locks are not economically repairable. If the case is worn, corroded or has been forced, a like-for-like replacement into the existing mortice is a straightforward job with the old lock as a template.
- Never force a key that has stopped.If the key turns partway and jams, take the pressure off and try turning back. Wiggle it very gently while turning back. Force at this point breaks the key inside a lock that then has to be dismantled.
If it doesn't work
Key turns but the bolt only comes partway out
Cause: Worn levers, a worn key, or an obstruction in the case — Fix: Try the spare key first. If the bolt still stops short, the case is worn and the lock needs replacing — a bolt that does not fully throw is not securing anything.
The bolt will throw but not withdraw
Cause: The bolt is under load from a door bearing against the keep — Fix: Push or pull the door hard while turning the key to unload the bolt. Then fix the alignment before it happens on a night when nobody is around to push.
Suddenly stiff after redecorating
Cause: Paint in the keep, on the forend, or run into the keyhole — Fix: Clean the keep and forend with a scraper, and clear the keyhole. This is why locks and keeps come off before painting rather than being cut in around.
Everything is stiff in the whole house
Cause: Humidity — timber doors have all swollen together — Fix: Seasonal. Improve ventilation, and if it recurs every year, seal the door edges properly rather than easing them repeatedly.