How to Fit a Key to a Bit-Key Mortise Lock

A Skeleton Key.
A Skeleton Key. | Source

Before Reading Further...

First try a skeleton key and see if it works in your lock. At right is one kind of skeleton key. Its design is based on the assumption that the bit of the key that is supposed to work your lock has no end cuts and the double-curve design of the bit will pass all wards that might otherwise prevent it from turning. Also the bit is thin enough that it will pass most warded keyholes. You can find skeleton keys like this and other varieties as well at many hardware stores and locksmith shops. One of them might work your lock.

If no skeleton key works, read on and decide if this is a project you would like to undertake or if you would rather find a professional locksmith to do the job.

Tools Needed

In order of appearance:

  • Safety Glasses
  • Screwdriver
  • Utility Knife
  • Vice
  • 4-inch Warding File

A Dremel Rotary Tool might also come in handy, and sometimes a small flat bastard file is good for roughing in cuts before you clean them up with the warding file.

External anatomy of the bit key lock.
External anatomy of the bit key lock. | Source

What this article is not about: This article is not about springs. This kind of lock will not work correctly if the springs inside the lock case are broken or worn out. So if your springs need replacing, do that before proceeding to making a key. Failure to do so could result in a nasty lockout that would likely involve sawing the bolt off while the door is locked shut.


Step one in the process of fitting a key is to remove the lock from the door.

  1. Remove the door knobs and spindle
  2. Remove the mounting screws from the lock face (the part of the lock that is only visible when the door is open)
  3. Standing facing the edge of the door (opposite the hinge edge), trace around the edges of the faceplate with a utility knife to free it from adhesion to the door's paint or varnish
  4. Insert a thin screwdriver or other metal rod not more than 1/4 inch in diameter through the spindle hole and through the other side of the door.
  5. Grasp the screwdriver handle and blade and pull gently but firmly.

The lock case should pop out of the door edge enough so that you can grasp it and pull it out.

WEAR EYE PROTECTION.

Set the lock case on a clean, flat surface. You'll notice one side has screws. Remove them, then carefully remove the cover to expose the inside of the lock case. With any luck springs will not fly out of it. Usually they stay put in this kind of lock.

Nashua Lock Company bit key mortise lock, ca. 1902
Nashua Lock Company bit key mortise lock, ca. 1902 | Source
Interior side of lock case cover showing wards.  Inset shows wards in red for visibility.
Interior side of lock case cover showing wards. Inset shows wards in red for visibility. | Source
Internal parts of a bit key mortise lock.
Internal parts of a bit key mortise lock. | Source
Fig. A
Fig. A
Fig. B
Fig. B
Fig. C
Fig. C

Bit Key in Motion

The first illustration at right is of the interior of the Nashua Lock Company bit key mortise lock shown in the two photos above. The cover of this lock has "wards" cast into it on either side of the keyhole (see photo with inset). The wards allow only those keys that are cut properly to pass them, so keys not so cut will not turn past the wards and therefore will not operate the lock.

In the photo, the wards on are on the cover, but in my illustrations at right I have placed them on the opposite interior wall of the lock case so that I can show how the key is affected more clearly.

The second illustration, Fig. A, shows the profile of the key during insertion and also turned 1/8 revolution. The rotated key is in the process of passing the ward.

It is possible to grind off the wards so that less cuts need be made in the key, but the wards provide stability as well as security, so the key may work less crisply if the wards are ground away.

Fig. B shows the bit of the key contacting the locking lever. A spring exerts constant downward pressure on the locking lever. The key raises the locking lever so that the bolt can pass the gap in the locking lever. It is possible to file the locking lever to fit a key, however this is not the best way to fit a key. Remember, while it is relatively easy to remove material from a piece of metal, it is fairly difficult to put it back.

Fig. C shows the locking lever raised and the bolt in the process of being pushed through the gap in the locking lever by the bit of the key. When the bolt is fully thrown, the user continues to turn the key until it aligns once again with the keyhole and can be withdrawn. The spring-loaded locking lever then drops back to its former position, closing the gap so that the bolt cannot move. This feature makes the bolt very difficult to force open. To do so usually results in the destruction of the lock.

The parts of a bit key.
The parts of a bit key. | Source

The Anatomy of a Bit Key

  • The Bow: grasped by the hand of the user, used to turn the key
  • The Shaft: this is self-explanatory
  • The Stop: aligns the key with the locking lever(s) and keeps it from being inserted past that point
  • The Bit: this part of the key interacts with the lock, lifting the locking levers and throwing or retracting the bolt.

Bit Key Blanks


To cut a key, first you need a key blank. A key blank is a key with no cuts made in it. Bit key blanks are not available everywhere, but some locksmiths carry them. Your best bet is to bring your lock body to the locksmith shop and check out key blanks to see if they will go into your lock. The location of the stop is also important. If a blank goes in it is important that it does not go in too far. It is not such a problem if it goes in too little. You can cut away part of the bow side of the bit so that it will fit inside the lock case. It is also not much of a problem if the bit is too wide and hangs out of the other side of the lock case. That excess material can be cut away. Ideally, when you are done, the tip of the key should protrude out of the other side of the lock no more that 1/8 of an inch or so.

Similarly if you see that the bit is too tall for the blank to enter the keyhole, but otherwise the blank would go in, this is not a problem. You can file material off the end of the bit until the blank is small enough to go in.

If a blank is too thick, it is a bit of work to make it thinner. It is best if the key blank is thin enough to insert before you begin cutting it.

Once you have found a key blank you think you can make work in your lock, buy 6 to 10 blanks so that if you ruin a couple you don't have to take the walk of shame back to the locksmith shop to buy more. They are not cheap, so perhaps you can make a deal with the locksmith to take back the ones you never take a file to.

If no blanks are available, often bit keys from other locks can be made to work in your lock with a little modification. Often you can find orphaned bit keys at yard sales and antique shops.

Types of Bit Key Cuts
Types of Bit Key Cuts | Source
Plain Keyhole vs. Warded Keyhole
Plain Keyhole vs. Warded Keyhole

Cutting the Key

The first illustration at right shows the various cuts that can be made into the bit of a bit key.

  • End cuts are generally made to accommodate locking levers. The cut must be made to the correct depth that will raise the locking lever to the correct height to allow the bolt to pass at the correct point in the rotation of the key. The best method would be to measure the distance and then cut the key accordingly. Usually I just file a little and try the key, file and try the key, etc., until the bolt passes the locking lever without resistance.
  • Side cuts are made to accommodate wards cast into the lock case wall or lock case cover as illustrated above in the "Bit Key In Motion" section.
  • Ward cuts are made to accommodate wards cast into the actual keyhole (see second illustration at right). These wards are designed to keep keys without the proper ward cut from being inserted into the lock. There is usually no problem with simply filing off the keyhole ward, thereby making the warded keyhole into a plain one. Look carefully, however. Sometimes these wards are part of a larger scheme of wards inside the lock, making for a very complex key. If the innards of your lock looks like the Nashua Lock Company example I have used here, you needn't worry about that.

In all you do, file mindfully and try your work often in the open lock body. Hold the internal parts in place with one hand while you observe the operation of your key.

Filing an end cut.
Filing an end cut. | Source

I used to enjoy fitting keys to bit key mortise locks. It feels good to bring something old and disused back to life and usefulness; it feels good to see the fruit of your patient, mindful efforts when the job is done and the lock works well.

As you can see, however, it is a rather involved, time consuming process. Locksmiths fully equipped for this kind of work have milling machines with special vises that allow them to make the end cuts, side cuts and ward cuts with ease. You will have to decide if you wish to take this journey or delegate it to a professional.

© 2013 Tom Rubenoff

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