Lock Only Cylinder Modification: Caution!

[Raymond]

I recently read a write up in a popular trade magazine about this design modification. Perhaps I am missing a crucial design point and am asking to discuss this for clarification. They used a Kwikset deadbolt on which the key turns one way to lock and the opposite way to open. The key cannot turn a full 360 degree turn.

The design to modify a standard pin tumbler lock to allow the plug to turn in one direction only by one key and either directions with another key has been around for many years. This is accomplished by milling a flat area on the plug from one bottom pin hole out so as to make a step tapering back up to the full diameter of the plug. The 'locking' key is cut deeper in this space so that the bottom pin is raised only as high as the bottom of the step. The plug can turn in one direction only.

Wonderful!

But what happens when the original key is used and turned the wrong direction first or turns to lock? This key raises this pin to the top of the step (full plug diameter). It can rotate smoothly with the shear line in the opposite direction from the 'lock only'. When it is turned in the direction of the 'lock only' key, the top pin can drop down into the milled flat and will not allow the plug to turn back. The top pin will be hitting the side of the bottom pin raised to the full plug shear line. The plug is now "locked up" as the plug can not go back, can not go around forward and the key is not removable.

Am I missing something or is this a faulty design modification?

Thoughts?

[Tyler J. Thomas]

It's called a "Brink's function" and Schuyler Towne did a write up about making his own many years ago on here. I know it's stickied too.

I did a cursory glance of the article you mention; I'll read it again Monday to see if I can't help answer your questions. I can't believe they published that article - it looks like a complete hack job and I would hope that no one in their right mind would sell a solution like that, even temporarily, to a customer.

[Raymond]

This idea has been bugging me so I applied my thinker and have maybe figured out how to make it work. I still don't like the concept.

The slot at the side of the pin hole must be at an angle and stay very short. It would help to make the bottom pin like a Kwikset with a heavily beveled edge. This way there will not be enough room for the top pin to actually fit at the side of the full size bottom pin and the top pin can ramp up this bottom pin and slide over the gap.

[dll932]

I made a Brink cylinder just to see how well it worked. No problem I could see. I made one key that turns both ways, one that only goes CW, another that only goes CCW.