Customer Key Breaks in Lock One Week After Rekey: Post-Service Decision Tree

Why this matters

A key that snaps inside a cylinder one week after a rekey will almost certainly be attributed to the rekey by the customer, regardless of the actual cause. Responding without a structured diagnosis means either accepting liability for a failure that was not yours or dismissing a legitimate quality issue with the work. This tree walks from the break location to the probable cause, covers key extraction, and addresses what to document regardless of where liability lands.

Step 1: Extract the broken key fragment

Before diagnosing the cause, retrieve the fragment. Do not attempt to grip the fragment with needle-nose pliers; pliers enlarge the keyway throat and can deform the cylinder housing, making extraction harder and turning a simple recovery into a cylinder replacement.

Correct extraction method: apply a small amount of PTFE dry-film spray or graphite to the keyway opening. Use a hook pick or a dedicated broken-key extractor, inserting alongside the fragment and engaging the serrations. Work the fragment out with small back-and-forth strokes rather than a single pull. In tight keyways a second extractor on the opposite side of the blade can provide the leverage needed to walk the fragment out.

Retain the fragment. The break location and break character are the primary diagnostic data.

Step 2: Read the break location

Examine where on the blade the break occurred. The location is the most reliable single indicator of cause.

Break at the bow (the handle or head end of the key): this location is not load-bearing during normal cylinder operation. A bow break indicates the key was bent by mechanical stress outside the lock, most likely from a heavy keychain applying lever force at the bow or from a pre-existing crack. This is not a rekey fault. Document the bow-break location in the work order and explain the external stress pattern to the customer.

Break at the tip (the first few cuts from the end of the blade): tip breaks are the most common post-rekey failure point. The tip is the narrowest cross-section of the blade and is also the first part to enter and rotate into the bottom of the cylinder chamber. A depth-of-cut mismatch concentrates bending stress at the tip on every use. One cut that is too deep relative to its neighbor creates a thin web of metal between the high and low cuts; repeated bending at the shear line fatigues that web and it fractures at the tip.

Break in the mid-blade: mid-blade breaks share the same depth-of-cut mechanism as tip breaks but are less common. They can also indicate a wrong blank with incorrect shoulder stop positioning, which puts the entire blade under lateral stress on every insertion.

Step 3: Assess the cylinder after extraction

Once the fragment is out, test the cylinder with a freshly cut working key from your code machine. The cylinder should operate smoothly through its full range of motion. If it does, the cylinder is not the primary problem and the key was the failure point.

Check the pin stack: insert the working key and rotate slowly. If you feel intermittent catch points that were not there before, a pin may have been nicked by the broken fragment during rotation. Remove the cylinder, inspect and replace any damaged pins, and retest.

Branch 1: Bow break

Cause: external mechanical stress, pre-existing fatigue, or keychain overload. Not a rekey fault.

Action: provide the customer with a freshly cut replacement key from the original plug pinning. Explain that cutting from your code-machine record rather than duplicating the existing cut prevents bitting drift; a duplicate of a duplicate made on worn-cut equipment accumulates small depth errors on each generation, and those errors concentrate stress at the thinnest blade points.

Branch 2: Tip or mid-blade break with depth-of-cut mismatch

Cause: one or more cuts in the bitting sequence are at an incorrect depth, creating a high-low stress concentration. This can result from a worn cutter wheel on the key machine, from cutting from a worn duplicate rather than the original code, or from a bitting error on the rekey.

Action: pull the rekeyed cylinder and verify the pin heights against the bitting code on file. A bottom pin that is one increment too tall seats the key blade too deep in that chamber and bends the blade slightly on every rotation. Correct any mismatched pins and recut the keys from the corrected code. Replace the cylinder if chamber damage from the broken key is found. Document the corrected pin stack and new key cuts in the work order.

Branch 3: Wrong blank family used for the cylinder

Cause: a blank from an incompatible family has a different shoulder stop position than the cylinder calls for. When the shoulder stops at the wrong depth, the entire blade sits in the keyway at the wrong height relative to the pin chambers. The lateral load on every use fatigues the blade faster than a correctly positioned blank would.

Action: identify the correct blank family from the cylinder manufacturer's specification, cut a new set of keys from the correct blank, and verify shoulder stop position with a depth-and-spacing gauge before completing the job.

Documentation regardless of liability

Record the break location, the branch diagnosis, any corrected pin heights or blank family corrections, and the key cuts provided to the customer. If the break is a bow break and not a cylinder fault, note that explicitly. If you made a correction to the pin stack or blank, note that as well. Clear documentation protects against a follow-up dispute weeks later when the details are gone.

References

• ANSI/BHMA A156.5, Cylinders and Input Devices for Locks, current edition.
• ALOA Institutional Pin Tumbler Service Guidelines (bitting depth tolerances and blank identification).
• ALOA Key Cutting Standards and Code Reference, current edition.
• Door and Hardware Institute (DHI), Builders Hardware Reference Guide (key blank family and shoulder stop specifications).