Cross-Referencing a Lock You Already Buy

Look at the search terms that bring people to a hardware factory's site and a pattern appears immediately: a great many of them are another company's model numbers. A rim lock from one brand, a concealed hinge from another, a latch code off a twenty-year-old carton, a part reference from a catalogue that is out of print. Nobody types those hoping to read about us. They type them because they have a part that works, the supplier is gone or slow or expensive, and they need to know what to order to get the same thing again.

It is a good question and it has a precise answer, which is that a lock is defined by its geometry and not by its name. Two locks with the same six numbers are interchangeable regardless of what either is called. Two locks with the same name and one different number are not.

For a mortise lock, those six are: backset, from the face of the door to the centre of the spindle; centre distance, between the spindle and the cylinder or keyhole; faceplate length and width; case height and depth; spindle size, almost always 8mm square but worth confirming; and door thickness range. Get those six and the substitution works. Miss the centre distance and the handle and the cylinder will not both line up with the holes already cut in the door — and the door is the thing you cannot change.

For a rim lock or night latch the list is shorter but the same in character: backset, case size, whether the cylinder is single or double, latch extension, and the strike type, because a rim strike and a rim deadbolt keeper are different castings and the frame is already prepared for one of them.

For a hinge: leaf height and width, knuckle count, pin type, and whether it is a fixed pin or loose. A hinge that is 3mm taller than the mortise already routed into the frame is a hinge that does not fit, and nothing about the finish or the grade changes that.

What does not carry across, and should be ignored when cross-referencing: the model number itself, the finish name, and the grade designation. Finish naming is not standardised between factories — one supplier's satin nickel is another's brushed nickel is another's SN — so match a finish by looking at a sample rather than by matching a word. Grade designations are standard-specific: an ANSI grade and an EN classification measure different things under different tests and neither translates into the other by arithmetic.

The practical route, and it is short. Measure the part you have, or photograph it beside a ruler with the faceplate visible. Send those numbers, the door thickness, and whether it is a left or right hand door. Our engineering desk will come back with the model that matches, or — and this is the part worth saying — with a straight answer that we do not make one. A near-miss recommended as an equivalent is how a container of unusable metal happens, and we would rather lose the line than send it.

One more thing, for anyone replacing a part in an existing building rather than specifying a new one: check the strike and the keep before you order the lock. In a retrofit the frame is the constraint, not the door, and the item that most often makes a substitution fail is not the lock body at all — it is the strike plate cutout, which was made for the old part and will not be remade.