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Q: Our old doors still have their antique locks, but no keys. Can I get keys that work?

—Margaret Cowie, Stark, N.H.

Master locksmith Larry Cosares replies: Of course you can! And, most likely, one key is all you'll need because interior doors on old houses typically share identical locks.

The first step is to remove one of the locks and take it in to an antiques store or a locksmith that has a collection of old bit keys, often called skeleton keys. If you're lucky, you'll find a key that actually slides into the keyhole and throws the bolt, and your quest will be over.

If not, look for a bit-key blank that's a close fit, and follow the steps shown here to cut the key so that it operates the lock. It's a simple job with a nice reward: bringing an old lock back to life.

Shown: TOH senior technical editor Mark Powers had this bit key made to get his old door locks working again.

Step 1

Remove The Lock

Photo by Kolin Smith

Loosen the set screw on one door knob and twist or pull it off the spindle, then pull the other knob and the attached spindle out of the hub. Cut through the paint around the faceplate and remove the screws, as shown. Stick the screwdriver through the spindle hub and pop the lock out of the door.

Step 2

Fit The Tip to The Keyhole

Photo by Kolin Smith

Find a blank with a tip diameter that fits into the top of the keyhole on each side of the lock. If the key's rectangular blade—called a bit—goes in and turns but doesn't throw the bolt, go to Step 5. Otherwise, proceed to Step 3.

Step 3

Fit The Bit to The Keyhole

Photo by Kolin Smith

If the bit is too long, mark where it hits the bottom of the keyhole. Using a file or a bench grinder, as shown, remove just enough metal from the bit's lower edge for the bit to pass through the keyholes on both sides of the lock.

Step 4

Fit The Bit to The Lock Case

Photo by Kolin Smith

Now check the bit's width by resting the swollen knuckle of the shaft—the stop—against the edge of the case, as shown. (The tip must project past the other side.) Mark where the bit's front and back edges need trimming for it to turn inside the case. File those edges to fit.

Step 5

Work The Bolt

Photo by Kolin Smith

Remove the cover of the lock case. Using your thumb to hold the locking lever against the bolt, insert the key from below, and turn it, as shown. The bit should push the lever out of the way and retract the bolt. If it hangs up on the bolt, file a shallow notch in the bit's bottom edge so that it turns freely in both directions.

Step 6

Notch The Bit's Edges

Photo by Kolin Smith

Inside the case's cover, alongside the keyhole, look for tiny bumps, called wards. (See Step 7 photo.) Pass the bit through the keyhole and mark where it contacts the wards. Then insert the key's tip from the opposite direction and mark where the wards hit the bit's other edge. Clamp the key in a vise and file two notches in the bit, as shown.

Step 7

Test The Turn

Photo by Kolin Smith

Insert the key through the keyhole from below, as shown, and spin the bit. The notches should pass freely over the wards when you twist the key in either direction. Flip the key around and test the notches on the opposite side, too.

Step 8

Reassemble and Replace

Photo by Kolin Smith

Dust out the lock's case and spray it with a penetrating lubricant, such as WD-40, as shown. Replace the cover and make sure the key throws the bolt from both sides. Reinstall the lock in the mortise, screw on the faceplate, and replace the spindle and knob. Then put your key in a nice safe place.