Pin tumbler

How a lock gives itself away

A pin-tumbler lock is a brass plug that wants to turn inside a housing. Stacks of pins cross the gap between them: a key pin below, a driver above, a spring on top. The right key lifts every stack until the split between its two pins sits exactly on that gap, the shear line, and the plug turns.

No lock is drilled perfectly. Turn the plug a little with a tension wrench and the chamber that is furthest off true pinches its pin first. Only that pin binds; the rest stay springy. Lift the binding pin until its driver clears the shear line and the plug jumps a fraction of a degree. That jump is the click, and it traps the driver on the plug’s edge. Then another pin binds.

Push a set key pin on up past the shear line and it wedges there: overset. Let go of the wrench and everything drops. Too much tension and nothing moves at all.

Security pins

A spool driver has a narrow waist. When the waist reaches the shear line, the plug swings further than a set pin could turn it: a false set. Lift that spool through and its lip pushes the plug back against your wrench, so you have to ease off just enough to let it pass. A serrated driver has grooves that catch the plug’s edge one after another, each one a click that isn’t the set.

At the bench

• Set the tension with the wrench control, the scroll wheel, the bracket keys, or the digits 0 to 9
• Press below a pin and push up to lift it; slide across to move the pick between pins
• Or use the arrow keys: left and right step between pins, up and down lift and lower, Shift for finer steps
• Your best time on each lock is kept in this browser

Pick only locks you own, or have been asked to open.