If you have an older house with knobs that slip, latches that are stuck inside the door, or other minor problems, then you should follow these instructions to remove the lock, dis-assemble it, clean and perform minor repairs.
Step 1: Meet the mortise lock and door knobs.
This mortise lock set is installed in a 1930 colonial in the Boston area.
Notice the set screw on the base of the knob on the right. This screw should hold the knob securely to the spindle when you turn the knob.
Step 2: What's a spindle?
The spindle is threaded to hold the knobs on, but has flats to operate the lock mechanism and to allow the set screws in the knob to hold the knob securely to the spindle.
If the set screws become lose they will allow the knob to turn without actuating the lock mechanism. If your knob is "slipping" try tightening this screw.
Prolonged use with a lose set screw can damage the spindle. If tightening the set screw does not help, you should be able to buy a new spindle at your local hardware store (not the chain with the 16 year old clerk, find the one with the proprietor who is approximately as old as your home).
Step 3: Loosen the set screw.
To remove a knob, find the set screw on the base and loosen it just enough that the handle turns freely on the spindle.
Step 4: Remove a knob from the spindle.
Unscrew that knob from the spindle. Make sure not to drop it!
Step 5: Remove the spindle from the door.
To remove the spindle, pull on the other knob with the spindle attached.
If you need to replace the spindle loosen the set screw on this knob, and unscrew the spindle from the knob to remove it.
Reverse your steps from this point to install the new spindle.
Step 6: Free the lock face from any paint.
Before you remove the lock mechanism from the door, you may want to score the gap around the lock front with a razor to free any paint that may be attached to the metal.
If there is paint on the lock face, then when you remove the mechanism the paint may stick to the lock face and peal off the wood of the door.
Step 7: Remove the lock face screws.
To free the lock mechanism from the door, remove the two screws in the lock face.
Step 8: Pop the lock case out of the door.
To extract the lock mechanism from a tight mortise hole, place a small screw driver through the hole where the spindle was and gently push forward to free the lock face from the wood.
Step 9: Remove the lock case from the door.
The lock mechanism should come freely out of the door.
Step 10: Open the lock case.
Beware that several components inside the case may be attached to springs. If you are not careful when you open the case, you will not get a chance to see how the inner workings are supposed to fit together, or worse parts, may fly off into the far corners of your workspace.
Carefully remove the screw while holding the case together.
Step 11: Freeing the latch
If you need to reverse the latch so that it closes the other way, gently remove the latch while holding the other parts in place, and re-insert it with the angled end facing the other direction.
If that's all you needed to do, reverse your steps from this point to re-install the lock.
However, look at all those cobwebs! Lets continue on to clean the lock.
Now would be a great time to take a photo of your lock mechanism so you can put it back together later.
Step 12: Disassemble the latch mechanism
The latch/knob mechanism should not have particularly powerful springs, just be careful not to lose any of the pieces.
Step 13: Disassemble the dead bolt mechanism.
The deadbolt mechanism will have a powerful spring. Press the gate toward the top of the lock mechanism, lift it over the top of the cam on the deadbolt , and gently release the spring tension until you can safely lift the gate off it's pin.
Step 14: Keep track of the parts!
I warned you about losing parts, right?
Step 15: Secure the lock case.
The main lock body should be cast and fairly durable. The lock face is likely brass and is therefore easy to dent and even bend. Use a vise to hold the case body so you can clean the face, but do not over-tighten the vise or clamp directly to the brass face, or you will damage your lock.
Step 16: Polish the brass lock face.
Use 0000 steel wool to polish the lock face. Do not think you are clever and use sand paper to rush the job along. Be patient. Remember you are here because some lazy sod didn't bother ot remove the lock before painting. Look at all the gunk on there! Do not repeat their mistake. Patience!
Step 17: Decide what to do about dings and dents.
You may reveal marks from the craftsman who built your lock. All mine have unique numbers stamped into the faces.
Deep dents or gouges in the brass will resist polishing. You can decide to try to clean them out but I just leave them dark. They add character.
Step 18: Polish the exterior latch and dead bolt surfaces.
The latch and possibly even the deadbolt will also be polished brass. You can clean them with steel wool in the same way you cleaned the lock face. I don't put them in the vise to polish, I just hold them in my hand.
Step 19: Clean the case and parts with WD-40.
If you did it right, your lock case will be full of steel wool dust. Clean it and all the other parts with a paper towel and WD-40.
Do not use oil!
Get all the dust, cobwebs, and steel wool out of the case and off the parts.
Step 20: Re-install the deadbolt and prepare the gate.
The first tricky part of re-assembly is to get that gate back on with that stiff leaf spring. With the deadbolt in place, reverse your steps: gate on pin, leaf spring against the stop, gently squeeze the gate into position...
Step 21: Attach the gate to the dead bolt.
...and drop it onto the cam of the deadbolt. Tension should keep it in place.
Step 22: Replace the latch spring.
If your latch spring is broken or weak then you'll need a replacement.
You can take the lock to a local locksmith (find the one that's been around since your house was built).
If you have a good spring (maybe in a different lock) you can measure the length of the spring when un-tensioned. Then hang weights from it to determine the amount of force needed to start stretching it (the spring pretension force) and then amount of additional force needed to stretch it a specific distance (the stiffness of the spring) then you can go find one online. The McMaster-Carr site has a great search engine for springs.
My locks took this spring: http://www.mcmaster.com/#9654k116
Step 23: Prepare the latch mechanism.
The latch mechanism can be tricky since the top of the case normally holds it in place. I put the parts together as much as possible with the spring un-tensioned.
Step 24: Tension the latch spring.
I use a screw driver to tension the latch spring and hold everything into place.
Step 25: Use the screw driver to hold everything in place.
Notice that the case top is already on the screw driver so I can drop it into place.
Step 26: Close the case top.
Once the case is closed, the latch mechanism should stay in place while you tighten the case screw.
Step 27: Test the mechanism on the bench before re-installing.
Do not over-tighten the case screw or it will bind the mechanism. Test by using a screw driver to actuate the mechanism. If the latch doesn't come back out when you release the screw driver, then you've either over-tightened the screw, or something else is out of place inside.
If you have a key for the lock, also test the deadbolt.
Now is your chance to fix things, if you don't test it until you have it back in the door, you'll be sorry when you have to tear it all back out and bring it back to the bench.
Make sure your latch is the right way too, if it's backward fix it now.
Once you're satisfied, reverse your steps to re-install the lock set in the door. If the case doesn't easily slide in to the door, use a rubber mallet to gently tap it into place. Do not use a metal hammer or you will mar the surface of the lock face. If you do not have a mallet use the plastic handle of your screw driver.
Step 28: Note that there are many kinds of locks.
Note that not all mortise locks are the same inside. I have two kinds of lock mechanisms that look almost identical from the outside but have a slightly shorter face and a slightly shallower backset.
The latch mechanism on this lock is easier to work with because it doesn't fly apart when you open the case. There's a leaf spring for the knob return and a separate coil spring for the latch return. Also the the spindle cam pushes directly on a combined arm/transit.