Rekey Pin Stack Pressure Feedback Decision Tree

Why this matters

A rekey is the most frequently performed service in a locksmith's workflow, and most failures during reassembly trace to one of three causes: pin stack height that does not match the depth chart, spring pressure that is too high or too low for the plug to rotate freely, or a binding plug from a misaligned or contaminated shell. A new locksmith pins a cylinder, drops it back in the lock, and the key will not turn. The branches below let a tech read the failure mode from the plug's tactile feedback and resolve which subsystem to recheck. The decision tree assumes a standard pin tumbler cylinder; high-security cylinders (Medeco, Mul-T-Lock, Abloy) follow their own procedures.

Symptom presentation

After pinning, the key insertion or rotation gives feedback in four distinct ways. The key inserts smoothly but the plug binds at full rotation, the key inserts but the plug binds before any rotation, the key inserts and the plug rotates but the cylinder feels gritty or "false-sets" at every position, or the key cannot fully insert. Each feedback points to a different cause and a different fix.

A bound plug at full rotation usually indicates a driver pin sitting too high in the shell, blocking rotation. A bound plug at the start of rotation usually indicates a key pin that is too short and the driver pin is not crossing the shear line. A gritty feel with false sets indicates pin-to-pin misalignment, often from picking debris or from a pin stack with mismatched diameters. A key that will not insert usually indicates a debris in the keyway or a key pin sitting below the keyway floor.

Quick checks at the bench

Before reassembly, verify each pin stack against the depth chart. For a 5-pin Schlage C cylinder using SC1, the key pin lengths come from Schlage's depth chart (depth 0 = 0.335 inch, depth 9 = 0.200 inch in 0.0150 inch increments). The driver pins are uniform standard (usually 0.230 inch). The total stack height (key pin + driver pin) should equal the shear line height plus the desired depth, with a 0.005 inch tolerance.

Inspect the springs. Stock pin tumbler springs are standardized but spring fatigue from repeated rebuild or from a long-storage cylinder weakens them. A weak spring lets the driver pin sit below the shear line and the plug never deadlocks. A spring with a permanent compression set should be replaced.

Inspect the plug for burrs. A rebuilt or cleaned plug should rotate freely in the shell without the pins installed. If the plug binds without pins, the cause is mechanical, not pinning.

Isolation tree

Branch 1 - plug binds at full rotation. A driver pin is too long or has not been seated against the spring. Common causes: wrong depth chart consulted, a key pin substituted with a similar but not identical length, or a driver pin from a different manufacturer's kit. Pull the cylinder, identify the binding stack with a pin tray and a follower tool, and verify against the chart. Reseat or replace the offending stack.

Branch 2 - plug binds before any rotation, key inserts cleanly. A key pin is too short. The driver pin sits below the shear line at rest, and inserting the key pushes it back up but not enough to clear the shear line. Verify the key pin length against the chart for that cut depth. If the key pin is correct but the plug still binds, the issue may be a worn or out-of-spec key (see worn-key decision tree).

Branch 3 - cylinder feels gritty or false-sets. Pin-to-pin misalignment or debris between the plug and shell. Disassemble, blow out the chambers with compressed air, and reassemble with a small drop of light dry lube (graphite or PTFE-based; never wet oil, which collects grit). False sets often resolve with cleaning alone.

Branch 4 - key will not insert fully. Either a debris in the keyway (a piece of a previous key snapped at the tip), a key pin sitting below the keyway floor (improper assembly with the key pin upside down), or a fundamentally wrong blank. Verify the blank is the correct profile for the cylinder. Pull the cylinder and inspect the keyway floor and pin chamber for debris or a misplaced pin.

Branch 5 - cylinder rotates but the lock does not unlock. The tailpiece is not engaging the bolt. This is a lock-side issue, not a pinning issue. Verify the tailpiece is correctly oriented relative to the keyway. Schlage cylinders typically use a horizontal tailpiece; Kwikset uses a vertical. A cylinder rotated 90 degrees from its intended orientation will appear to "not work" even when the pinning is perfect.

Confirming diagnosis

The follower tool is the most diagnostic instrument in pin tumbler work. With the plug pulled and the followers in place, drop each key pin into its chamber from above and feel the stack height. The top of the key pin should sit at the shear line when the correct cut depth is in place. A pin that sits below shear line is too short; a pin that protrudes above shear line is too long. The visual and tactile check resolves most pinning errors in seconds.

For spring pressure, compress each spring in the chamber with a pick or a small punch. A new or like-new spring offers consistent resistance through its full travel and returns the driver pin to its rest position firmly. A weak spring offers little resistance and lets the driver pin drift. Replace weak springs whenever a cylinder is rebuilt; they are cheap insurance.

For plug-shell fit, the plug should rotate smoothly with light finger pressure when the cylinder is empty. Binding without pins indicates a mechanical fault in the plug or shell (a burr, debris, or a corroded surface) that pinning cannot resolve.

Remediation

For pin-stack errors (Branches 1 and 2), verify against the depth chart, swap the offending stack, and retest. Carry a full pin kit on the truck for the major manufacturers; running out of a specific depth pin is the most common reason a rekey takes longer than it should.

For grit and false sets (Branch 3), clean with compressed air, replace weak springs, lubricate with dry lubricant, and reassemble.

References

• ANSI/BHMA A156.5 American National Standard for Cylinders for Door Locks and Latches - the standard governing pin tumbler dimensions and grades.
• ANSI/BHMA A156.30 American National Standard for High Security Cylinders - the standard for high-security cylinder rekey procedures.
• UL 437 Standard for Key Locks - the security listing referenced for high-security and commercial cylinders.
• Manufacturer depth-spacing and pinning charts (Schlage, Kwikset, Yale, Medeco, Mul-T-Lock, Sargent, Best) - the reference data for pin length and stack assembly.
• ALOA (Associated Locksmiths of America) Master Locksmith reference materials - field references for rekey procedures and pin tumbler service.