Springs Weak but Working: Replace Now vs Monitor Condition Decision Tree

Why this matters

Pin-tumbler springs degrade gradually. They start at full coil height, lose tension, take a set, then eventually fail to drive top pins reliably and the lock no longer keys consistently. Catching them in the middle of that arc is the call you have to make on most service visits. Replace too early and you pad invoices for parts that had service life left. Replace too late and the customer is calling you back inside the warranty window. This tree gives you the gates that turn a judgment call into a documented decision.

Step 1: Identify why you are looking at the springs

Springs are rarely the headline complaint. You usually find weak springs while diagnosing something else: a sticky cylinder, a rekey, a callback on key feel. Note the entry path on the ticket because it affects the threshold:

• Headline complaint was binding or sticking: weak springs may be the root cause, replace.
• Headline complaint was unrelated, springs found incidentally: apply the condition gate below, do not auto-replace.
• Cylinder is on a master-keyed system: tighter threshold, replace earlier because master systems are more sensitive to spring tension drop.

Step 2: Visual condition gate

With the cylinder open and pins pulled, lay the springs on a clean white surface next to a fresh new spring of the same part. Compare:

• Free length within 10 percent of new: full life remaining, monitor.
• Free length 10 to 25 percent shorter than new: aged, replace if you are already in the cylinder, monitor if you are not.
• Free length more than 25 percent shorter than new: replace, the spring has taken a permanent set.
• Visible kink, bent coil, or rust: replace regardless of length.
• Coil-to-coil spacing uneven: replace, the spring is buckling under load.

Step 3: Functional gate

A spring can read short on free length and still drive its pin stack acceptably. Test function before condemning purely on appearance:

• Drop the spring on top of its top pin in an upright chamber. The pin should be driven fully home with the spring barely compressed.
• Press the pin down with a follower and release. The spring should return the pin to seated position with no hesitation.
• With cylinder reassembled, operate with the key 30 cycles. The plug should return to the rest position from any rotation under spring tension alone, with no pin staying up.

Failure on any of these means replace, regardless of free-length reading.

Step 4: Cylinder context gate

Apply the visual and functional gates inside the context of the cylinder:

• Single-cylinder residential deadbolt, low cycle count: monitor borderline springs.
• Commercial cylinder on a high-cycle door (front entry of a retail space, employee entry): replace borderline springs proactively, the cycle count will catch up fast.
• Restricted or high-security cylinder (UL 437, BHMA Grade 1 high-security): replace, the security premium does not pay if the springs are halfway through their life.
• Cylinder pulled for rekey: replace as part of the rekey, marginal cost is low and you avoid a return trip.
• Cylinder pulled for cleaning only, no other work: monitor unless gates above failed.

Step 5: Customer and warranty gate

The decision is partly business, not just mechanical:

• You are inside a recent service warranty: replace, the cost is yours either way.
• You have a service agreement covering preventive work: replace, that is what the agreement pays for.
• This is a one-time service call and the customer has declined preventive add-ons: document the spring condition on the ticket with photos, recommend in writing, and respect the customer's choice if they decline.

Never replace springs silently and add the line item. Always offer, document, and price.

Step 6: Replace as a set

When you replace, replace every spring in the cylinder, not just the worst one. Mixed spring ages introduce inconsistent pin-stack tensions and produce uneven feel across rotations. The marginal cost of the full set is small.

For master-keyed cylinders, replace all springs at once across the cylinder being serviced, but do not reflexively pull every cylinder in the system. Stagger system-wide spring refresh into a preventive maintenance plan instead.

Step 7: Document the monitor case

When you choose monitor over replace, write it on the ticket explicitly. Note free-length percentage, cycle context, and the next service interval at which you recommend reassessment. A six-month note on a residential deadbolt or a three-month note on a high-cycle commercial cylinder is a reasonable default.

Send the customer a copy. A documented monitor decision protects you if the spring fails between visits and the customer claims you should have caught it.

Step 8: Stock for the call

Carry a spring assortment on the truck that covers your top three keyway families and BHMA grade ranges. The replace-now decision is meaningless if you have to schedule a return visit to install. Springs are pennies in inventory and dollars in callback avoidance.

References

• ANSI/BHMA A156.5, Cylinders and Input Devices for Locks
• ANSI/BHMA A156.30, High Security Cylinders
• ALOA Pin Tumbler Service and Preventive Maintenance Guidelines
• Builders Hardware Manufacturers Association, Care and Maintenance of Architectural Hardware