Bottom pin

The bottom pin — also called the key pin — is the pin in a pin-tumbler chamber that the key blade directly contacts. When the key is inserted, the bottom pins ride on the cuts of the key, each lifted to a height determined by the depth of the cut at that position.

Each cut depth corresponds to a specific bottom-pin length. A position-3 cut at depth 4 means a bottom pin of length 4 (in profile-specific units) sits in chamber 3. The deeper the cut, the longer the bottom pin needed to reach the shear line.

Bottom pin sizing

Bottom pin lengths come in discrete units defined by the lock profile:

• Lockwood 570: 10 depths (0–9), 0.025″ increment, factory MACS 4
• Schlage C: 10 depths (0–9), 0.0125″ increment, factory MACS 7
• BEST A2 SFIC: 10 depths (0–9), 0.0125″ increment, MACS 6

When a cylinder is pinned at the bench, the locksmith reads the change-key depths off the pinning chart and selects bottom pins of the matching length from a pin kit. A standard kit holds bottom pins from 0 (the shortest) up to 9 (the longest) plus driver pins and master pins in similar gradations.

Bottom pins in master keying

In a mastered cylinder, the bottom pin sits at the depth of the deeper of the two keys it must accept. The master pin sits above it, sized to fill the gap between change-key depth and master-key depth. The driver pin completes the stack.

So for a chamber where the change key cuts at 6 and the master key cuts at 4 (master key is the higher key), the bottom pin is length 4, the master pin is length 2, and the driver pin tops out the stack to the standard total height.

Related

→ Pin tumbler — the mechanism the bottom pin lives in → Shear line — what the bottom pin must reach → Driver pin — the pin sitting above the bottom pin → Master pin — the pin between bottom and driver in mastered chambers