Installing a mortise lock on a commercial door requires cutting a rectangular pocket (mortise) into the edge of the door, inserting the lock case into the pocket, and then installing the trim, key cylinder, and thumbturn. Mortise locks are the strongest lock type used in commercial applications and are designed to withstand high-traffic, high-abuse environments. This guide covers the complete installation procedure for a standard commercial mortise lock on a hollow metal or wood door.
CDF Distributors ships complete door, frame, and hardware assemblies from its Nashville, Tennessee headquarters. You can configure and order assemblies with mortise locks online using CDF’s ProBuilder tool at cdfdistributors.com. For assistance selecting the correct mortise lock function for your project, call (855) 769-9895 or email sales@cdfdoors.com.
What Is a Mortise Lock?
A mortise lock is a lock mechanism that is installed inside a rectangular pocket (called a mortise) cut into the edge of a door. The lock case, which contains the latch bolt, deadbolt, and operating mechanism, slides into this pocket and is secured with a faceplate that sits flush with the door edge. Trim components including levers or knobs, a key cylinder, and a thumbturn are then mounted through the door face and connected to the lock case.
Mortise locks require a pocket in the edge of a door, into which the lock case is fitted. Though more costly than cylindrical locks, this type of lock offers greater durability and strength. A mortise lock is the strongest, most adaptable lock and is designed to handle high-traffic, high-abuse applications. Mortise locks are standard in schools, hospitals, government buildings, and other institutional facilities where security and longevity are required.
Installation Specifications
This guide covers the installation of a standard commercial mortise lock into a 1-3/4” thick commercial door (hollow metal or wood). The procedure applies to common ANSI/BHMA A156.13 mortise lock cases. Specific dimensions vary by manufacturer and lock function. Always refer to the manufacturer’s template for exact mortise pocket dimensions, hole locations, and backset measurements before cutting.
At a Glance
Lock Type | Mortise lock (ANSI/BHMA A156.13) |
Door Thickness | 1-3/4” (commercial standard) |
Typical Backset | 2-3/4” (standard commercial) |
Mortise Pocket | Rectangular cutout in door edge sized to lock case |
Components | Lock case, faceplate, trim (levers/knobs), key cylinder, thumbturn |
Door Material | Hollow metal or wood (solid core) |
Fire Rating Compatibility | 20-minute, 45-minute, 60-minute, 90-minute, and 3-hour rated assemblies |
Installation Time | Approximately 45 to 90 minutes per door |
Tools and Materials
- Mortise lock template: manufacturer-supplied template for marking hole and pocket locations
- Tape measure: for verifying door thickness, backset, and layout dimensions
- Pencil or scribe: for transferring template markings to the door edge and face
- Drill with assorted bits: for boring cylinder hole, thumbturn hole, and mortise pocket relief holes
- Hole saw (1” to 1-1/4”): for boring the key cylinder and thumbturn holes through the door face
- Wood chisels (1/4”, 1/2”, 1”): for squaring the mortise pocket corners and cutting the faceplate recess (wood doors)
- Mallet: for driving chisels during mortise pocket preparation
- Router or mortising jig: for removing material from the mortise pocket efficiently (optional but recommended)
- Angle grinder with cutoff wheel: for cutting the mortise pocket in hollow metal doors
- File set: for smoothing and finishing the mortise pocket edges
- Screwdriver set (Phillips and flat): for securing the faceplate, trim, cylinder, and thumbturn
- Allen wrench set: for set screws on trim and cylinder collars (sizes vary by manufacturer)
- Level: for verifying plumb alignment of the lock case in the door edge
- Safety glasses: required PPE during cutting and drilling operations
- Hearing protection: required PPE when using power tools on hollow metal doors
- Dust mask: required PPE when routing or drilling wood doors
Step-by-Step Installation
Step 1: Review the Manufacturer’s Template
Every mortise lock ships with a paper or metal template that specifies the exact dimensions and locations for the mortise pocket, cylinder bore, thumbturn bore, and faceplate recess. Before making any cuts, read the template thoroughly and confirm that the template matches the lock model you are installing.
Identify the following measurements from the template:
- Mortise pocket height, width, and depth
- Backset distance (typically 2-3/4” for commercial mortise locks)
- Cylinder bore diameter and center height from the bottom of the lock case
- Thumbturn bore diameter and center height
- Faceplate dimensions
Confirm that the door thickness is 1-3/4”. Mortise locks for commercial doors are designed for this standard thickness. A door that is thinner or thicker will require different hardware.
Step 2: Mark the Door
Position the template on the door edge at the correct height. Standard lockset height places the centerline of the lever or knob at 38” to 40” from the finished floor, though this may vary based on project specifications or ADA requirements.
Tape or hold the template firmly against the door edge and transfer all markings:
- On the door edge, mark the top and bottom of the mortise pocket, the faceplate outline, and the centerline.
- On the door face (lock side), mark the center of the key cylinder bore.
- On the door face (opposite side), mark the center of the thumbturn bore.
Double-check all markings against the template before proceeding. Misplaced holes cannot be corrected without replacing the door.
Step 3: Cut the Mortise Pocket
The mortise pocket is the rectangular cavity cut into the door edge that receives the lock case. The method for cutting depends on the door material.
Wood Doors
- Drill a series of overlapping holes along the centerline of the mortise pocket to the required depth using a drill bit that matches the pocket width. Use a chisel and mallet to remove the remaining material and square the corners. A mortising jig or router with a straight bit can speed this process and produce cleaner results.
- Test-fit the lock case in the pocket. The case should slide in snugly without forcing. Remove additional material with a chisel or file if needed.
Hollow Metal Doors
- Use an angle grinder with a cutoff wheel to cut along the marked outline on the door edge. Cut carefully to avoid exceeding the pocket boundaries.
- File all edges smooth. Hollow metal edges can be sharp and must be deburred for safe handling and proper lock case fit.
- Test-fit the lock case. The case should insert fully with the faceplate sitting flush against the door edge.
Cutaway diagram showing: mortise pocket in door edge, lock case inserted, faceplate flush with door edge, and cylinder/thumbturn bore locations through door faces. This diagram requires CDF approval before inclusion in the production file.
Step 4: Bore the Cylinder and Thumbturn Holes
Using the hole saw at the diameter specified by the template (typically 1” to 1-1/4”), bore the key cylinder hole through the door face on the exterior or corridor side. Bore from one side only partway, then complete from the opposite side to prevent blowout and splintering on wood doors.
Bore the thumbturn hole on the opposite face of the door at the location marked from the template. The thumbturn hole is typically the same diameter as the cylinder hole.
After boring, clean all holes of debris and test-fit the cylinder and thumbturn components to confirm alignment with the lock case.
Step 5: Cut the Faceplate Recess
The faceplate must sit flush with the door edge. On wood doors, use a chisel to cut a shallow recess matching the faceplate dimensions and thickness. On hollow metal doors, the faceplate recess is typically already formed by the mortise pocket cut.
Place the faceplate over the pocket opening and trace its outline if not already marked. Remove material to the exact depth of the faceplate thickness so it sits flush with the door edge surface.
Step 6: Insert the Lock Case
Slide the mortise lock case into the pocket with the latch bolt and deadbolt facing the strike side of the frame. The faceplate should sit flat against the door edge with no gaps.
- Secure the lock case by driving the faceplate screws into the pre-drilled holes. Tighten evenly to avoid canting the faceplate.
- Verify that the latch bolt retracts and extends smoothly by pressing the latch tail or operating the hub with a screwdriver.
- Verify that the deadbolt throws and retracts fully.
Step 7: Install the Key Cylinder
Insert the key cylinder through the cylinder bore hole from the exterior or corridor side of the door. The cylinder tailpiece must engage the lock case’s cylinder hub.
- Thread the cylinder into the lock case or secure it with the cylinder collar and set screw from the interior side, depending on the manufacturer’s instructions.
- Test the cylinder by inserting the key and turning. The latch or deadbolt should operate smoothly. If the cylinder tailpiece is too long, it may need to be trimmed to the correct length.
Step 8: Install the Thumbturn
Insert the thumbturn assembly through the bore hole on the interior side of the door. The thumbturn spindle must engage the deadbolt hub inside the lock case.
Secure the thumbturn with its mounting screws or set screw. Test the thumbturn by rotating it to confirm the deadbolt throws and retracts fully.
Step 9: Install the Trim
Mount the lever or knob trim on both sides of the door according to the manufacturer’s instructions. Trim installation typically involves:
- Placing the interior and exterior escutcheon plates (roses) over the cylinder and thumbturn holes.
- Inserting the spindle through the lever hub in the lock case.
- Mounting the levers or knobs on each side and securing with set screws or through-bolts.
- Tightening all fasteners and verifying that both levers return to the horizontal position when released.
Step 10: Install the Strike Plate and Test
Mark the strike plate location on the door frame by closing the door and marking where the latch bolt and deadbolt contact the frame. Cut a mortise in the frame jamb for the strike plate and the bolt pockets.
Secure the strike plate with the provided screws. Test the complete assembly:
- Latch bolt retracts when either lever is operated
- Deadbolt throws and retracts with key and thumbturn
- Latch bolt engages the strike plate when the door closes
- Deadbolt aligns with the strike plate pocket
- Door closes and latches without binding or excessive force
- Both levers return to horizontal when released
Common Mistakes to Avoid
Avoid these common errors during mortise lock installation:
- Cutting the mortise pocket without using the manufacturer’s template. Generic measurements may not match the specific lock case dimensions, resulting in a pocket that is too large, too small, or misaligned.
- Boring cylinder or thumbturn holes at the wrong height. A misplaced bore will prevent the cylinder tailpiece or thumbturn spindle from engaging the lock case. This error typically requires door replacement.
- Failing to test-fit the lock case before installing trim. If the pocket needs adjustment, it is easier to correct before the trim is mounted.
- Over-tightening the cylinder set screw. Excessive torque can bind the cylinder and prevent key operation.
- Neglecting to check deadbolt alignment with the strike plate. A deadbolt that does not fully engage the strike pocket provides reduced security.
- Installing the lock case upside down. The latch bolt and deadbolt have specific orientations. Verify that the latch bolt is positioned at the correct height relative to the lever trim before securing the faceplate.
- Skipping the faceplate recess on wood doors. A faceplate that protrudes past the door edge will prevent the door from closing flush against the frame stop.
Mortise Locks in Fire-Rated Assemblies
Mortise locks are compatible with fire-rated door assemblies at all five standard rating levels: 20-minute, 45-minute, 60-minute, 90-minute, and 3-hour. In a fire-rated assembly, the lock, door, frame, and all hardware components must carry matching fire labels and be listed for use together. Verify all code requirements with the project specifications and the Authority Having Jurisdiction (AHJ).
Configure Your Assembly
CDF’s ProBuilder tool allows contractors and facility managers to configure complete door, frame, and hardware assemblies online. During configuration in ProBuilder, mortise lock function, trim style, and finish are selected as part of the complete assembly. ProBuilder displays compatible options and provides instant pricing.
For assistance selecting the correct mortise lock for your application, call (855) 769-9895 or email sales@cdfdoors.com. CDF’s sales team is available Monday through Friday, 7:30 AM to 4:30 PM CT.
Frequently Asked Questions
What is the difference between a mortise lock and a cylindrical lock?
A cylindrical lock (also called a bored lock) is installed by boring two circular holes in the door. A mortise lock is installed inside a rectangular pocket cut into the door edge. Mortise locks contain the entire latch and deadbolt mechanism in a single case, offering greater durability and strength than cylindrical locks. Mortise locks are designed for high-traffic, high-abuse applications such as schools, hospitals, and government buildings. Cylindrical lever locksets are the most commonly used lock type and are suitable for a wide range of commercial applications where the added strength of a mortise lock is not required.
What door thickness is required for a mortise lock?
Standard commercial mortise locks are designed for 1-3/4” thick doors, which is the commercial standard for both hollow metal and solid core wood doors. Residential doors (typically 1-3/8”) require different hardware. Confirm door thickness before ordering.
Can I retrofit a mortise lock onto a door that currently has a cylindrical lock?
Retrofitting is possible but requires significant door modification. The existing bored holes must be filled or covered, and a new mortise pocket must be cut. In most cases, it is more practical to replace the door with one prepared for a mortise lock. Consult with CDF’s sales team to determine the best approach for your project.
What mortise lock functions are available?
Mortise locks are available in numerous functions to suit different access control requirements. Common functions include entrance/office (locked and unlocked by key, with a deadbolt operated by thumbturn), classroom (locked and unlocked by key from outside only), storeroom (always locked, requires key to enter), and passage (latch only, no locking). CDF’s ProBuilder displays all available functions during assembly configuration.
How long does it take to install a mortise lock?
An experienced installer can typically complete a mortise lock installation in 45 to 90 minutes per door. Installation time depends on the door material (hollow metal versus wood), whether the door is pre-prepped, and the complexity of the trim. Doors that are factory-prepped for mortise locks require significantly less field time.
Do mortise locks work with fire-rated doors?
Yes. Mortise locks are compatible with fire-rated door assemblies at all five standard rating levels: 20-minute, 45-minute, 60-minute, 90-minute, and 3-hour. All components in the assembly must carry matching fire labels. CDF ships fire-rated assemblies with labels riveted to each component.
About This Guide
This installation guide reflects CDF Distributors’ standard hardware specifications and field-tested procedures used by commercial contractors nationwide. CDF’s technical team reviews and updates all installation documentation to ensure accuracy against current ANSI/BHMA standards and manufacturer installation requirements.
For questions about mortise lock installation or to order a complete door assembly with a mortise lock, contact CDF at (855) 769-9895 or sales@cdfdoors.com. Configure your assembly online at cdfdistributors.com using ProBuilder.
