Mortise Locks: The Lock Case Built Into the Door

Mortise vs. Bored Locks

The difference is in how the lock goes into the door.

• Bored lock. ANSI/BHMA A156.2 describes the Series 4000 bored lock as one "installed in a round bored opening in the edge and face of a door." Schlage's ND Series template, for example, calls for a 2-1/8-inch cross-bore.
• Mortise lock. A mortise lock has a rectangular case that slides into a deep pocket in the door edge. Trim, cylinders, and a thumbturn attach through smaller holes in the door faces.

Allegion's support article on mortise locks rates them "medium, high to extra heavy duty" for commercial, multi-family, and institutional use. It lists "most functions available (40+)" as a strength and higher cost as the main drawback. For the bored alternative, see cylindrical locks.

Anatomy of the Lock Case

A typical commercial mortise lock combines several parts in one body. Schlage's L Series catalog gives concrete figures for one widely specified line:

• Case. Cold rolled steel with zinc dichromate plating. The L9000 case measures 4-7/16 × 6-1/16 × 1 inch.
• Backset. 2-3/4 inches (70 mm) only. Backset is the distance from the door edge to the center of the trim.
• Door thickness. 1-3/4 inches standard.
• Front. The LIST dictionary defines the front as "the portion of a lock which provides the means of mounting the locking device to the edge of the door." An armored face plate "conceals all lock and/or cylinder mounting screws."
• Bolts. Mortise locks can carry a latch, a deadbolt, and an auxiliary deadlatch in the same case. A156.13's F04 Entry Lock function, for example, combines a latch bolt operated by lever from either side with an "auxiliary dead latch."
• Trim. Levers or knobs drive the latch through spindles. Escutcheons or roses cover the door face.

Because the lock body is separate from its trim and cylinders, many mortise lines can be set up for different jobs. Schlage describes the L Series as "built on a universal lock case." It also lists a "field reversible latchbolt" among its features, so the latch can be turned for door handing on site. See lock functions and door handing.

How the Cylinder Mounts

Most North American mortise locks take a threaded cylinder. The LIST dictionary defines a mortise cylinder as "a threaded cylinder typically used in mortise locks of American manufacture." ANSI/BHMA A156.5 adds that it is "regularly supplied with a cam to actuate the lock mechanism," and gives a typical thread size of 1.150 in.–32 UNS-2A. The cylinder screws into the case. BEST's installation instructions for its mortise cylinders describe a cylinder clamp screw in the front edge of the lock that holds the cylinder in place.

Mortise locks can also take interchangeable cores. Schlage's L Series supports conventional, full size interchangeable core (FSIC), and small format interchangeable core (SFIC) cylinders. Allegion's listing shows that the choice can change the certified security grade:

• L/LV9000 with conventional cylinders: A156.13-2005 Series 1000, Grade 1 Operational and Grade 1 Security
• with FSIC cylinders: Grade 2 Security
• with SFIC cylinders: Grade 3 Security

Outside North America, many mortise locks are made for euro profile cylinders instead. See rim and mortise cylinders and interchangeable cores.

ANSI/BHMA A156.13

ANSI/BHMA A156.13-2022 covers mortise locks and latches. BHMA's summary says it includes "operational tests, strength tests, security tests, cycle tests, finish tests and dimensional criteria." Examples from that summary:

• Operation. Lever-operated locks must open with a maximum torque of 28 in-lbf.
• Durability. Grade 1 locks must pass 1,000,000 cycles of opening and closing with a 10-pound load applied.
• Security. Fifteen tests are specified, including a 1,200 in-lbf locked lever torque test and two directions of impact.
• Fire doors. Hardware for fire doors is evaluated and listed to UL 10C by an accredited third-party laboratory.

For comparison, ANSI/BHMA A156.2-2022 also sets 1,000,000 cycles for Grade 1 bored locks. At Grade 1, the two standards have the same cycle count, so grade, not lock format, is the fair comparison. See ANSI/BHMA lock grades.

Fire ratings are listed by product. Schlage states that the L Series is UL listed for use on 3-hour fire doors, except the L9076 and L9077 functions. See fire-rated door hardware.

BS 3621 in the UK

In the UK, BS 3621 (current edition BS 3621:2017+A1:2024) specifies thief-resistant lock assemblies operated by key from both the inside and the outside of the door. Assemblies with keyless egress, or with keyless egress plus a secure mode, are covered by BS 8621 and BS 10621.

BSI's own description is careful about scope:

• BS 3621 covers a "single-point lock assembly," including locking plates, fixing screws, fitting instructions, cylinders, and protective furniture where appropriate.
• The lock in that assembly must conform to BS EN 12209.
• The standard covers performance, not a single lock format. It is not specific to lever locks.

BSI separates the keyless-egress standards "because they are sufficiently different in terms of what they offer for means of escape (from fire) and security."

A Little History

A Locksmith Ledger industry history says Corbin introduced "the first field-reversible without disassembly mortise lock" in the 1930s. It says Russwin's Ten Strike followed in the 1950s, "a soon-to-be universal mortise lock that brought about standardization of the size of the mortise lock face," with internal parts that could be converted to different functions. Those claims come from a single trade-press source and are attributed here accordingly. The same history notes that when many new makers entered the cylindrical lock field, each with its own sizes and shapes, locksmiths were "routinely called on to re-mortise doors for new locks."

Related Topics

• Cylindrical locks: the bored alternative
• Lock functions and door handing
• Rim and mortise cylinders, euro cylinders, and interchangeable cores
• Latches, deadbolts, and strike plates
• ANSI/BHMA lock grades
• Fire-rated door hardware and panic hardware
• Multipoint locks
• Lever handles and accessibility
• Rim locks

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