- The shear line is the boundary between plug and housing. A key works by splitting every pin stack there.
- Binding on insertion points to spacing. Smooth insertion with no turn points to depth.
- A lock that works only with the worn original has worn to fit that key.
- Dry lubricant only. Oil collects dust and creates a paste inside the cylinder.
The shear line
A pin tumbler cylinder has a rotating plug inside a fixed housing. Drilled through both are five or six chambers. Each chamber holds a spring, a driver pin above and a key pin below. With no key in the lock the springs push the drivers down so they cross the gap between plug and housing, and the plug cannot turn.
The correct key lifts each key pin exactly to the point where the top of the key pin meets the bottom of the driver pin at the boundary between plug and housing. That boundary is the shear line. With every stack split at that line there is nothing crossing the gap and the plug rotates freely.
Everything else follows from this. A key cut too shallow leaves a driver still crossing the line. A key cut too deep pushes a key pin across it instead. Both feel identical to the customer, which is a dead key, and both are diagnosed by looking at cut depth rather than by replacing the lock.
Depths, spacing and why a key sticks
A key is described by two things. Spacing is the distance from the shoulder to each cut position, and depth is how deep each cut is. Manufacturers publish both as a depth and space chart. Kwikset uses seven depths on a 0.023 inch increment. Schlage uses ten depths on a 0.015 inch increment, which is why Schlage keys look more finely stepped.
A key that goes in but binds is usually a spacing error, because the pins are not sitting at the bottoms of the cuts. A key that goes in smoothly and will not turn is usually a depth error. Learning to separate those two symptoms saves an enormous amount of guesswork.
The ramps between cuts matter too. If adjacent cuts differ by too many depth increments the ramp becomes a cliff, the key pin cannot climb it, and the key jams. Every manufacturer publishes a maximum adjacent cut specification for exactly this reason and a code cutting machine respects it automatically.
Wear, and the lock that only works one way
Cylinders wear predictably. Brass key pins mushroom at the tip, springs weaken, and the plug face gets a lip worn into it by years of key insertion. The classic symptom is a lock that works with the original key and not with a fresh duplicate, which sounds backwards until you realize the cylinder has worn to fit that one worn key.
Another common one is a lock that turns from inside but not outside on a double cylinder deadbolt, which usually means debris or a broken pin on the outside plug rather than a failed bolt.
Repair beats replacement more often than customers expect. New springs and pins, a clean out with a dry lubricant and a fresh key to code will restore most residential cylinders. Never use an oil based lubricant inside a cylinder. It collects dust and turns into a paste that guarantees a return visit.
Common Questions
What actually stops the lock turning?
The driver pins bridge the gap between the plug and the housing. Until every pin stack is split exactly at that gap, the plug is pinned in place. The right key lifts each stack to the same line at once.
Do more pins mean a better lock?
Slightly, but the pin count matters far less than the pin design. A five pin lock with spool pins and a tight plug resists picking better than a six pin lock with standard pins.
Why does my key work if I jiggle it?
Worn pins and a worn key leave enough slack that several positions will pass. It works until it does not, usually on the coldest night. Rekeying with fresh pins fixes it.