Mortise Vs Cylindrical Lock Door Prep
Why this matters
A door prepped for a cylindrical lockset cannot accept a mortise lockset without significant rework, and a door prepped for mortise has a pocket that cylindrical hardware does not fill cleanly. The decision between mortise and cylindrical drives the door prep on every commercial install, and an upgrade or replacement that crosses the family line is a planned-rework project, not a swap. This article gives the prep dimensions, the ANSI A115 family that standardizes them, and the field workflow that handles both prep types.
What each family is
Cylindrical (bored) lockset - the lock body is contained in a hole bored through the face of the door (typically 2-1/8 inch diameter), with a perpendicular bore for the latch and bolt mechanism through the edge of the door (typically 1 inch). Two trim escutcheons sandwich the door. Common in residential and light-commercial work. ANSI A156.2 grades cylindrical locks.
Mortise lockset - the lock body is contained in a rectangular pocket (mortise) cut into the edge of the door. The lock body is 4 to 5 inches deep, full lock-case width inside the door edge. Trim cylinders and thumbturns mount through the door faces into the lock case. Standard in commercial and institutional work. ANSI A156.13 grades mortise locks.
ANSI A115 prep standards
ANSI A115 (multiple parts) standardizes door preparation for various lockset types. The relevant prep standards:
- ANSI A115.1 - Door prep for cylindrical lockset 2-1/8 inch bore through the face.
- ANSI A115.2 - Door prep for mortise lockset, defining the mortise pocket dimensions and trim hole locations.
- ANSI A115.3 - Door prep for deadbolts (typically 2-1/8 inch face bore with 1-inch edge bore for deadbolt body).
- ANSI A115.4 - Door prep for electric strike installations.
A door from a commercial supplier specified to A115.2 has a mortise pocket cut, the front and rear cylinder holes drilled, the armor-front cutout machined into the edge, and the strike-side prep completed. The locksmith installs the lock body and trim; no field machining on a properly prepped door.
Cylindrical prep specifics
A residential or light-commercial cylindrical install has a cross-bore through the face (2-1/8 inch diameter, 2-3/8 inch backset residential or 2-3/4 inch backset commercial), an edge-bore through the edge (1 inch diameter, intersecting the cross-bore at the latch), a latch face mortise in the edge of the door (typical 1 inch wide by 2-1/4 inch long, 1/8 to 5/32 inch deep), and a matching strike mortise on the frame. A standard residential door slab from a home center comes pre-bored to A115.1 with 2-3/8 inch backset and the edge bore at standard latch height.
Mortise prep specifics
Mortise lockset prep is more involved:
- Pocket cut in the door edge - typically 6 inches tall by 4 to 5 inches deep by 1 inch wide (or larger for heavy-duty cases). The pocket is cut with a router or mortise mortiser, not drilled.
- Front cylinder hole - through the face, 1-1/4 inch typical diameter, located per the lock manufacturer's template.
- Rear cylinder hole or thumbturn hole - through the opposite face, located per the template.
- Lever or knob spindle hole - through the face, 1/2 to 3/4 inch diameter typical, located per the template.
- Armor front cutout - rectangular pocket in the door edge for the lock case's armored front plate.
- Strike pocket on the frame - larger than cylindrical strike preps because the mortise lock has a heavier latch and may have an auxiliary deadlatch.
Mortise prep is impractical to perform in the field on a wood door without specialized router jigs. Pre-prepped doors are the standard; field prep is reserved for high-skill applications on solid-core or institutional hollow-metal doors with the right tooling.
Compatibility constraints
The two families are not field-interchangeable without rework:
- Replacing a cylindrical with a mortise requires routing a new mortise pocket - difficult or impossible on hollow-core doors, time-consuming on solid-core. The cylindrical bores leave holes that must be plugged and refinished.
- Replacing a mortise with a cylindrical leaves a mortise pocket that the cylindrical hardware does not cover. A blanking plate (mortise-to-cylindrical conversion plate) can fill the visible pocket but is a cosmetic compromise and may not be code-acceptable on fire-rated doors.
The right path on a major hardware change is usually a new door slab with the appropriate prep, especially on fire-rated openings where any field machining must be done within the door manufacturer's specifications.
Hollow-metal door prep
Commercial hollow-metal doors are typically ordered with the prep already cut and reinforcement plates welded in place. Cylindrical prep uses 16 to 14-gauge reinforcement plates welded around the cross-bore and edge-bore. Mortise prep uses reinforcement plates that line the pocket and accept the lock case mounting screws. The strike prep on the frame gets a reinforcement plate behind the strike location. Field-modifying a hollow-metal door means cutting through the face skin into the door's internal reinforcement; if the reinforcement is wrong, the field cut delivers a non-functional opening. Always verify the prep designation on the door's spec sheet before ordering hardware.
Fire-rated door prep
Doors in a fire-rated assembly must be modified only within the listing of the door's manufacturer:
- Door manufacturer publishes a list of allowed preps per listing (cylindrical only, mortise only, both).
- Field machining beyond the listing voids the fire rating.
- NFPA 80 (Standard for Fire Doors and Other Opening Protectives) and the door's UL listing govern.
A locksmith doing a hardware upgrade on a fire door must read the door's label and the manufacturer's listing before cutting. An unauthorized prep on a fire-rated door is an inspection failure and a life-safety liability.
Field workflow - hardware replacement on existing prep
When replacing a like-for-like cylindrical or mortise: identify the existing prep dimensions (backset, cross-bore diameter, edge-bore diameter, latch face dimensions); specify replacement hardware that matches the existing prep; verify the new hardware's strike matches the existing frame prep; test fit before final installation; adjust strike alignment so the bolt or latch engages cleanly under the door's natural closing arc.
A hardware upgrade that crosses the cylindrical / mortise line on a fire-rated door is not a locksmith job alone. The door manufacturer must approve the modification in writing, or the door's UL label is void. If the original door cannot accept the desired hardware, the right answer is a new door from a labeled manufacturer with the correct prep - not a field workaround.
References
- ANSI A115 (multiple parts) - American National Standard Door and Frame Preparations.
- BHMA / ANSI A156.2 - American National Standard for Bored and Preassembled Locks and Latches.
- BHMA / ANSI A156.13 - American National Standard for Mortise Locks and Latches.
- NFPA 80 - Standard for Fire Doors and Other Opening Protectives.
- NFPA 252 - Standard Methods of Fire Tests of Door Assemblies.
- ALOA Security Professionals Association - Commercial Door Hardware Training.