Mortise Locks: Functions, Models and Replacement

A mortise lock is a lock whose steel case sits in a rectangular pocket cut into the edge of the door, with the lever or knob trim on both faces and a cylinder for the key. One case holds the latch and, on many functions, a deadbolt. We handle mortise lock replacement in Phoenix and across the Valley, and the first step is always working out what is on the door now.

A replacement goes smoothly when the new lock fits the existing pocket. Many American mortise locks share one prep, with a 2-3/4 in. backset and a 1-1/4 by 8 in. faceplate, so one maker’s lock can often replace another’s. Older or off-standard locks may need the door reworked, so a photo of the faceplate before we quote saves a surprise.

Why commercial buildings use mortise locks

A mortise lock is built heavier than a bored lever. The one case holds the latch, the deadbolt, the lever hub and the cylinder linkage, which gives more functions, a separate 1 in. deadbolt on many models, and parts that can be replaced without replacing the whole lock. They are common in schools, offices, hospitality and healthcare. Mortise locks are graded under their own standard (A156.13), so a Grade 1 mortise lock is not tested the same way as a Grade 1 bored lever.

The main mortise lines

Lines are not interchangeable at the trim level, and numbering differs by maker, so we confirm each model against the maker’s chart. For the bored-lock equivalent of this page, see commercial lever locks and their functions.

Mortise function numbers in plain words

Mortise functions carry ANSI/BHMA F-numbers, which are the common language across brands. Read the behavior, not just the name, because the standard uses the word “entrance” for several different functions. These are the ones we can describe reliably.

Cylinders and trim

The case, cylinder and trim are chosen separately, and many listings say “less cylinder” because the cylinder is ordered on its own. Most commercial mortise locks take a standard mortise cylinder, and many also accept interchangeable core housings, so the lock can follow the key system in the rest of the building (see IC core locks and master key systems). The cylinder’s length and cam must suit the lock. Trim comes in lever and knob styles, and it does not cross brands or even product families.

Door prep and the pocket

The pocket is a deep rectangular cut in the door edge, about 8 in. long, with a recess for the faceplate and a hole for the cylinder. Common American mortise locks share a 2-3/4 in. backset, a 1-1/4 by 8 in. faceplate and an ASA strike of 4-7/8 by 1-1/4 in., made for 1-3/4 in. doors. When the new lock follows the same prep, the existing pocket can often be reused.

When it does not, the door needs work. A different backset, case size or faceplate, or an older non-standard lock, can mean re-mortising, filling and redrilling, or changing the strike and trim holes. That is why swapping brands can need retrofitting, and why we look before we quote. A mortise door also cannot take a bored lock without reworking the door, and doing that in the field to a fire-rated door can compromise the rating. If the door carries a label, we ask first, and the building official has the final word. See commercial door lock replacement for how we handle rated doors.

Hand and backset

Hand is the side the hinges are on and the way the door swings, read from the outside. Many mortise cases, including the Sargent 8200 and Schlage L9000, can be reversed for the opposite hand, and the Schlage L9070 reverses without taking the case apart. Backset is the distance from the door edge to the center of the lever or knob hole, usually 2-3/4 in. Tell us both, or send photos and we measure on arrival.

Electrified mortise locks

An electrified mortise lock lets a card reader, keypad or remote release control the outside lever while the inside lever still lets people out. dormakaba’s E-PLEX and PowerPlex lines are examples, and their BHMA listings include Grade 1 for several of the functions above. The choices that matter most are fail-safe (unlocked when power is lost) or fail-secure (locked when power is lost), the wiring and power supply, and whether the door is fire-rated. See electric strikes and electrified locks and access control systems.

Rekeying a mortise cylinder

A mortise rekey usually changes the cylinder, not the lock case. The existing cylinder is re-pinned to a new key, or a new cylinder or interchangeable core goes in, and the case stays in the door. That is the usual way to cut off old keys when the lock itself is sound. Simple rekeys are priced on the phone. See commercial rekey and rekey locks.

Common failures

These are typical causes, not a diagnosis. We confirm the cause on the door.

• The key turns but the latch does not move. Often a loose or broken cylinder cam.
• The lever droops or wobbles. Often a worn spindle or trim, or loose screws.
• The latch or deadbolt sticks or does not extend. Possible causes are weak springs, a worn strike, or a door that has sagged out of line.
• The lock works from one side only. Possibly a wrong function for the use, or a damaged part inside.
• Too many keys are out there. Not a lock failure but a key control problem, and a rekey solves it.

With the trim off we check the lever return, the latch travel and the cylinder cam. That usually shows whether the fix is a part or the whole lock.

Repair or replace

Repair when the case is sound and the function is right: a cam, spindle, lever, strike or cylinder can be changed on its own. Replace when the case is worn, parts for the model are no longer available, or the function no longer fits how the door is used. Some makers publish conversion guides, and Schlage’s L-series guide shows how to turn a base L9050 case into other functions with added parts. Whether another maker’s case can be converted depends on that maker’s own guide. The options are a like-for-like body, a repair of the existing lock, or a move to a current line, and the model number on the faceplate tells us which are possible. For several doors, start a quote request.

How to identify yours

The faceplate on the door edge is the best place to start, and the model number on it, where there is one, points to the line and the function. The trim, the cylinder and a few measurements fill in the rest. If nothing on the lock is readable, a set of photos and the measurements below usually identifies it.

Aluminum and glass doors use different hardware, covered under storefront door locks. The wider section starts at the commercial locksmith hub.

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