Driver pin (top pin)

The driver pin — also called the top pin — is the spring-loaded pin above the bottom pin in each chamber of a pin-tumbler cylinder. The chamber’s spring pushes the driver pin down so that, when no key is inserted, the bottom pin sits at the bottom of the chamber and the driver pin straddles the shear line, blocking rotation.

When the correct key lifts the bottom pin to the right depth, the joint between the bottom pin and the driver pin lands precisely on the shear line. The driver pin retracts up into the housing’s chamber, the bottom pin sits inside the plug’s chamber, and the plug is free to rotate.

Driver pins and security

Driver pins come in a few shapes in addition to the standard cylinder:

• Standard — a plain cylindrical pin. Most commercial locks.
• Spool — the driver has a narrowed waist. When picked, the spool catches at the shear line in a way that tricks the picker into thinking the chamber has set when it hasn’t, raising pick resistance.
• Serrated — multiple narrowed sections along the length. Same idea, even more pick-resistant.
• Mushroom — a rounded head. Pick-resistance variant common on UL437 and high-security cylinders.

These security pin shapes don’t change the master keying math — the shear line is still a horizontal boundary at the same height — but they make the cylinder harder to pick or impression.

Driver pin sizing in master keying

Standard pinning practice keeps the total stack height in each chamber constant — bottom pin + master pin (if any) + driver pin = profile-specific total. A 6-pin Lockwood 570 chamber typically totals around 0.300″ depending on the cylinder body. Driver pins are sized to fill the gap left by the bottom pin (and master pin if mastered).

Related

→ Pin tumbler — the mechanism the driver pin is part of → Shear line — what the driver pin straddles when no key is inserted → Bottom pin — the pin directly below the driver → Master pin — the optional pin between bottom and driver