Master pin
An extra pin between the bottom pin and the driver pin that creates a second valid shear-line split, allowing two keys to open the same cylinder.
A master pin is a small additional pin placed in a cylinder chamber between the bottom pin and the driver pin (top pin). It creates a second valid shear-line split — one at the change-key depth, another at the change-key-plus-master-pin depth — which is what allows two different keys to open the same cylinder.
In an unmastered cylinder, each chamber has just two pins (bottom + top) and exactly one valid cut depth that aligns the shear line. To master a cylinder, you add a master pin and shorten the bottom pin and driver pin to keep the total stack height constant. The cylinder now opens at two cut depths.
Sizing rules
Master pins are sized in 1-increment units (matching the depth increments of the profile). On a Lockwood 570 with 0.025″ increments, a 2-unit master pin is 0.050″ thick. The thinnest master pins commonly used are 1-unit (0.025″) — anything thinner is structurally fragile and is flagged as a thin pin by quality grading.
Why master pins matter for security
Every master pin doubles the valid split points in a chamber. A 6-pin cylinder with one master pin per chamber has 2⁶ = 64 valid keys (2 splits × 6 chambers). Most of those 64 are phantom keys — combinations that open the cylinder but were never intentionally cut.
Reducing the number of master pins per chamber reduces phantom count. Allocation methods like the Rotating Constant Method achieve this by giving each change key one position at TMK depth — that chamber has no master pin, and the cylinder accepts only one split rather than two.
Related
→ Pin tumbler — the basic mechanism master pins extend → Shear line — the cylinder boundary master pins split → Thin pin — what happens when a master pin gets too small → Phantom key — the side effect of having more master pins