Lock Picking for Lock Makers

To design and build a lock the locksmith needs to understand the weaknesses of locks and how they are picked. This is not a lock picking manual.

Warded Locks

Even though warded locks reached a high degree of sophistication as an art form in the 18th Century they provided little security and often relied on trickery such as false keyholes, hidden keyholes and keyhole covers with hidden latches. There were two basic approaches to picking these locks. The first and most obvious was a simple tool or "false key" that was bent or shaped such that it passed the wards. Often this was a relatively simple shape compared to the complexity of the wards. Once the tool was wiggled past the wards it was twisted to lift the locking lever and shoot the bolt. The trick to this type picking of locks was to come prepared with a sufficient selection of bent picks to fit the lock. Often these were made after seeing the shape of the true key.

The second method of picking warded locks was to take an impression of the wards and make a pick or false key to fit. To take the impression the bit of a flat blank key of the correct size was covered with a layer of wax. The key was then carefully turned in the lock to make an impression of the wards in the wax. This was then used to make the pick or false key. In most cases this "skeleton key" had more material cut away then the true key to be sure it missed the wards. This produced a primitive looking tool but it worked.

To thwart the taking of impressions keyways were developed with shapes that in themselves were like the wards. To prevent a simple flat blank key from entering the keyway pins were introduced requiring barrel or "drill" keys. Then the portion through which the bit passed was made to be various shapes, particularly "S" shapes that took a carefully shaped blank key.

In the modern world where thieves no longer carry the relatively large and specialized picking tools needed in the past a good warded lock may present better security than it did 200 years ago. There are advantages to being unique or different. The greatest disadvantage of mass produced locks, no matter how well made, is that those who wish to circumvent them have greater opportunity to obtain samples to dismantle, develop tools and to practice upon them.

Lever Tumbler Locks

The best warded locks had a simple lever tumbler that prevented the bolt from moving without use of a key. The lever simply lifted out of a notch and the bolt could be shot. There was no security in the use of this device. It merely held the bolt in place so that vibration or the swinging of a door did not move it.

In 1778 Robert Barron of England invented the Lever Tumbler Lock where the levers had to be lifted to the correct height in order for the bolt to be shot. The levers could not be too high or too low. In the original Barron lock the bolt had the controlling slots and the levers had a dog or pin that protruded into the slot. For practical purposes there could only be two levers in this arrangement. Later the controlling slots were moved to the thin levers. This greatly simplified the lock and allowed the use of many lever tumblers. Cheap inside door locks had only two, but high security strong box and vault locks had as many as six lever tumblers.

The lever tumbler lock greatly improved the security of locks but they could still be picked. It merely required more skill and time. The time element is a great deterrent to a the if as they know that the longer they take the greater the likely hood of their being caught in the act. The method of picking a lever tumbler lock is very similar to picking the modern pin cylinder lock with the exception of the shape of the tools.

To pick a lever tumbler lock takes at least two tools. One is the tension device. These vary from a simple wrench to counter weighted lever arm. The purpose is to apply a gentle pressure to the bolt. The second tool is the pick used to lift the lever tumblers. Each lever is carefully lifted until its slot catches on the bolt's post.

Due to imperfections in manufacturing the levers the bolt will contact one before the others. By lifting on the lever that has the most resistance it will catch on the bolt as the post enters its slot by the same amount of imperfection that allowed the bolt to contact it first. This "traps" the lever in the slot. Then another lever is lifted until it is trapped and then another until the bolt can can be shot.

Another method, that requires more sophisticated tools, is to lift all the levers, then apply heavy pressure to the bolt sufficient to stretch the levers first contacted and hold all the levers in place. The pressure on the bolt is very gradually released dropping each lever and catching it on the post. This method does not always work but it has the benefit of being fast and simple, requiring less skill than picking by hand.

The more precision and smooth the parts of the lock the more difficult it is to pick. In a perfect lock the levers could not be trapped and thus the lock would be virtually impossible to pick. But nothing is perfect and it is the imperfection or manufacturing that allows locks to be picked. It is also important that the post and the slots in the levers have crisp square corners. A round post can be easily wiggled into the lever slots and lift the levers by force of the bolt even if the levers are misaligned.

One method that was used to make lever tumbler locks more difficult to pick was a series of teeth, like saw teeth on the end of the levers and on the matching face of the post. When the bolt is pressed against the levers it engages the teeth so that the levers cannot be lifted without backing off on the bolt. Thus a constant pressure cannot be applied and ever time the bolt is backed off there is a good chance that a trapped lever will be dropped.

It has also been found that it is best if the bit cuts in all the tumblers are the same shape and rest at the same height that it make it much more difficult to make a false key by trial and error fitting. When the bottom of the tumblers are different heights they give away the needed shape of the key.

A well made custom lever tumbler lock that combines warding and keyhole protection along with the increased security of a system of wards would be a very good lock in the modern world.

Detector Locks

The Chubb Detector Lock was and is one of the most secure lever tumbler locks invented. Although you hear reports of them and other high security locks being picked in competition you rarely hear the circumstances or the amount of time required. The circumstances are that the challenger had months to study and make tools specific to the lock and THEN took months to pick the lock. Yes, these locks CAN BE PICKED but any locking device that takes months (or even days) to bypass is more than good enough for almost all applications.

The Chubb detector lock makes picking much more difficult by trapping the bolt if any one of the levers is raised too high. While picking a lock each lever is tested to find the one that is contacting the bolt. If during testing the levers one is raised a very small amount higher than the minimum needed it trips the detector lever past the corner of the detector spring. This holds the detector lever UP so that the bolt cannot be moved unless the detector is reset by relocking the lock with the true key. Because the detector lever is help UP it can not be manipulated by picking since there is no way to pull the lever down. It IS possible to pick the rest of the levers and then reset the detector by over traveling the bolt in the locked position, but then the entire picking operation must start again.

IF there is a failed attempt to pick the Chubb Detector Lock the user of the true key would know immediately upon trying to unlock the lock. When the detector is tripped even the true key cannot unlock the lock without first relocking it and resetting the detector.

Chubb's Lock from 'On The Construction of Locks and Keys'
 John Chubb, 1850
 Digital Image Copyright © 2002 OldLocks.com
Jeremiah Chubb's lock, Patented 1818

The diagram above of Chubb's detector lock and the same description used by John Chubb in On The Construction of Locks and Keys, 1850 is repeated over and over again in both historical and technical references on Locks. The description is inadequate as it leaves a lot of questions.

K and F are the detector. K is the catch on the back lever and F the end of the horizontal detector spring. At rest the catch is under the detector spring the two overlapping by just a few thousandths of an inch. If the detector lever is lifted too high OR if any of the other levers are lifted too high raising the detector lever via pin G, the catch on the detector lever springs past the corner of the detector spring F which then holds the detector lever UP. Under normal operation the detector spring is lifted to just short of the point where it is tripped.

To reset the detector the levers must be properly aligned as in unlocking and the bolt's post B thrown into the over travel notch at I. This lifts the detector spring at F via the ramp on the bolt, thus releasing the detector. What is not shown is that there must be two cuts in the bolt for the key to enter and shoot the bolt similar to a double throw bolt. One is the normal cut that only shoots the bolt as far as needed for the levers to drop into place as shown. The second cut (to the right in the lock above) allows the key to throw the bolt the extra distance needed to reset the detector and then on returning (turning the key clockwise) it returns the bolt to the normal position so that the key can now enter the first cut and shoot the bolt in the normal manner.

Modern Chubb detector lock The other thing that is not shown is that the post cut out in the detector lever must have clearance for the post B to move while the lever is UP in the detected position to allow resetting the lock.

At left and below is a modern Chubb lock with the detector tripped.

Detail detector tripped.
Later, a device called a "curtain" was added to the Chubb's lock to increase security (see above). The curtain surrounded the key and had a front plate such that when the key, false key or picking tools were rotated the keyhole was covered except for the hole through which the shank of the key passed. Covers on the side of the key also narrowed the field of view to prevent easy inspection of the lock internals and make it more difficult to use picking tools.

It is an amazingly simple system that is relatively easy to manufacture while providing a very high level of security. Such a lock can also be hand crafted using simple tools to produce a custom lock.

Like most inventions there were constant improvements in the details of the Chubb's lock.

The author, Jock Dempsey, runs anvilfire.com and has produced a series on lock basics on the anvilfire iForge page.

Copyright © 2002 Jock Dempsey, OldLocks.com

[ Home | Links | Contact | Privacy Policy ]

Page Counter OldLocks GSC