A key rarely snaps without warning. It has usually been stiff for weeks, and one cold morning the extra quarter-turn finds the weak point.
Why it breaks at that exact spot
Keys fail at the shoulder, where the blade meets the head, because that is where the twisting force concentrates. Three things get them there:
- Wear. A twenty-year-old key has lost metal every time it has been used, and a key copied from another copy is worse still: each one is slightly off, and the lock has to be persuaded a little harder.
- A stiff cylinder. When a lock needs force, the key is the part that gives. Near the coast that stiffness is usually corrosion working into the pins.
- Leverage. Keys carried on a heavy ring, or used to pull a door closed, spend their lives being bent slightly.
Leave it exactly where it is
The half in the lock can often be gripped and drawn out with an extractor, and the cylinder saved. That is only true while the blade sits where it stopped.
Do not use pliers. There is almost never enough of the blade protruding, and what pliers reliably achieve is pushing the remainder deeper and rounding off the edge an extractor would have hooked.
Do not use glue. Superglue on a piece of wire is a popular trick. It does not hold against the friction of a cylinder, and now the pins are glued too, which converts a ten-minute job into a replacement.
Do not try the other half. Pushing the good half in after the broken one wedges both.
What an extraction involves
A locksmith aligns the cylinder so the pins are not gripping the blade, then hooks it out with a thin extractor down the keyway. Done properly it takes minutes and leaves the lock working.
Whether your remaining key still works afterwards depends on why it broke. If it snapped because it was worn, the other key in the drawer is probably worn too and is next in line. If it snapped because the cylinder is failing, the extraction is only half the job and the cylinder should be replaced while the technician is there.
When the cylinder cannot be saved
Sometimes the blade has already been forced deeper, the internals are damaged, or the cylinder had been on its way out for a year. Then it is drilled and replaced. That is more expensive, which is exactly why the first ten minutes after the break matter.
Stopping it happening again
A key that needs a technique is a warning. If yours has to be lifted, jiggled or leaned on, the lock needs attention before it chooses its own moment.
- Have the cylinder serviced with a proper lock lubricant. Not oil and not a household spray: both attract the dust that causes the problem.
- Retire keys that are visibly worn against a newer one.
- Take the key off a heavy keyring if you use it to pull the door shut.
Once the blade is out, a cylinder change or a service is usually the difference between this being a story and it being an annual event.