We are again finding more and more incorrectly rekeyed lock cylinders, recently “worked on” by a 3rd party locksmith or property handyman, using bottom pins as top pins. The calls for service come from customers who have found out that other keys, including the old keys, are working in the lock. This article discusses what we usually discover — the danger of bottom pins used as top pins in a key lock.

 

Brief Overview

 

A key cylinder is normally pinned to match a given key using bottom pins that have a conical point at one end of the pin. The pin shape allows the pin to ride the cuts on a key as the key is inserted or removed from the lock. Top pins are flat on both ends and are designed to fit into the cylinder with a spring on one end and the other end pressing flat and firmly on the bottom pin. When a cylinder is pinned with a bottom pin used as a top pin the conical shaped pin causes two thing to happen. The first is that the lock cylinder will operate extremely smoothly. But the second is that keys which should not work the lock will, in fact, open the lock. It also makes the lock very easy to pick open, even with a bent paperclip.

 

The cylinder in the photo shows one such bottom pin located in the top pin position. The lock in this case worked very smoothly AND/BUT a total of 1,296 other differently cut keys would also open the lock. The security level of this lock is basically zero.

 

For the more technically minded persons, continue reading the details below. Otherwise, at ApexAccess we check your locks for bad workmanship. We will replace the incorrect pins and will add pick resistant top pins on request. Your safety and security is our top priority. Give us a call to re-secure your property.

 

Normal Pin Tumbler Lock Operation

 

To understand the issue, let’s first recap how a standard pin tumbler lock works in a typical pin tumbler cylinder. This applies to most commonly used deadbolts or doorknobs. Some padlocks are also affected by the miss-use of bottom pins used as top pins.

 

• Bottom pins (also called key pins):

  These sit inside the rotatable plug (the part where the key inserts). They rest on the cuts of the correct key and have varying lengths to match those cuts. Bottom pins are usually flat on the top end (where they contact the top pin) and may have a slightly beveled or “bullet-shaped” (conical/rounded) end on the bottom to allow smooth key insertion without catching.

• Top pins (also called driver pins):

  These sit in the fixed housing (also call the bible) above the plug, pushed downward by springs. They are typically flat on both ends for stable contact—one end rests on the top of the bottom pin, and the other on the spring.

• Shear line:

  The break between the plug and housing. For the plug to rotate (unlocking the door), the correct key must lift each bottom pin so its top end aligns exactly at the shear line, pushing the top pin fully into the housing. The shear line measures in thousands of an inch, say .005 inches or less. It is called a clearance fit and is necessary to allow the plug to rotate freely within the lock cylinder assembly. The clearance is greater (larger/wider) in lower cost lock cylinders — in other words the shear line could be 0.10 inches, compared with a better made cylinder that operates with a tighter clearance (e.g. 0.005 inches or less).

 

In normal operation, the flat bottom of the top pin rests stably on the flat top of the bottom pin, creating a reliable, even contact surface. This ensures precise alignment only occurs with the exact key cuts.

 

The Mistake: Using Bottom Pins as Top Pins with Conical End Down

 

If a locksmith mistakenly (or incorrectly) substitutes bottom pins for top pins in the

housing — and orients them upside down so the conical/rounded (bullet-shaped) end points downward, resting on the actual bottom pin’s flat top, several mechanical and security problems arise:

 

• Unstable Pin Contact and Wobbling:
  Normally, flat-on-flat contact provides a broad, stable surface for the pins to rest against. With the conical end down, you get a point-to-flat contact: the narrow tip of the conical end presses into the flat top of the bottom pin.
  >This creates a tiny contact area (like a ballpoint pen tip on a table), allowing the “top” pin (the misused bottom pin) to tilt, rock, or wobble under spring pressure. The pin stack becomes unstable. The conical tip can easily slip or slide off-center when tension is applied to the plug. This action alone creates additional wear of the pin chamber (or cylinder housing) and leads to earlier failure.
• Easier Pin Setting and False Sets:
  During key insertion or picking, this instability makes it simpler for the pins to “set” at the shear line. The conical tip can more readily catch on the edge of the shear line or the bottom pin’s surface, mimicking a correct lift even if the key cut is slightly off. This leads to false sets (where pins seem set but aren’t fully aligned) or overly sensitive feedback. It frustrates legitimate users while making unauthorized manipulation (e.g., lockpicking or bumping) easier.

 

Security Vulnerability:

 

• Accepts Miss-cut or Unauthorized Keys:  The wobbly contact reduces the
  

  

  precision needed for pins to align. A key with shallow or incorrect cuts (e.g., a miscut master key or even a building common key) can now lift the pins just enough for the conical tip to “fool” the shear line, allowing the plug to turn. In practice, this can let wrong keys — like those from other doors in a multi-unit building — open the lock unexpectedly. For example, a tenant’s hallway key might unlatch a neighbor’s apartment door, creating a major breach in access control.

• Long-Term Wear and Reliability Issues: The concentrated pressure from the conical tip can accelerate wear on the bottom pin’s flat top, creating pits or grooves over time. Springs may compress unevenly due to the instability, leading to weaker downward force and a “mushy” or inconsistent lock feel. In extreme cases, pins could bind during rotation, jamming the cylinder.

 

Why Does This Happen?

 

This error might occur due to inexperience, using “universal” or mismatched pin kits (common in DIY rekeying), or confusing pin assortments during high-volume jobs. Quality lock pin manufacturers design bottom pins with the bullet shape specifically for keyway interaction, not for load-bearing contact with another pin. Swapping them disrupts the engineered tolerances (e.g., pin lengths and end geometries are optimized for their roles). A professional locksmith who deals with many different locks will often use a pinning kit designed for almost all re-pinning (re-keying) situations. The kit is not considered “universal” in the sense noted above.

 

 

Recommendation

 

One might think that the simple solution is to check a lock using any, or all, of the other keys that exist in their building. This approach has limited value. Other keys outside of your immediate location could work the lock. But without access to all of the possible keys the results are inconclusive.

As noted above, some keys will operate the lock but the action will seem mushy. In some cases a single key will begin to turn the lock but then meets resistance. Additional turning force overcomes the resistance. This is an indicator of either a worn key or an incorrect key used on a lock with the the upside-down bottom pins that we have discussed.</p>

 

The simple solution is to use a reputable commercial locksmith at all times. We can help.

 

At ApexAccess — your Locksmith in Colorado Springs — we check your locks for bad workmanship. We will replace the incorrect pins and will add pick resistant top pins on request. Your safety and security is our top priority. Give us a call to re-secure your property.