Mortise lock doors: which smart locks actually fit

A tubular deadbolt is a cylinder pushed through a round hole. A mortise lock is a steel box living inside a rectangular pocket carved out of the door edge, with the keyway threaded into the side of it. Every smart lock sold in a US home improvement aisle assumes the first arrangement. Owners of pre-war apartments, brownstones and old storefront-style doors tend to work this out somewhere between the product page and the moment the box is open on the hallway floor.

The door is not the problem. A mortise case is usually better hardware than the thing that replaced it in the 1970s, and there are three genuine ways to put electronics on one. Which of them is open to you gets decided by about eight measurements, worth taking before you shop rather than after.

Telling a mortise lock from a tubular one

Open the door and look at the edge. A tubular lock shows two small faceplates several inches apart, each roughly 1 inch by 2-1/4 inches, one for the latch and one for the deadbolt above it. A mortise lock shows a single tall plate, usually 1 to 1-1/4 inches wide and 7 to 8 inches long, with both bolts coming out through it: a square-ended deadbolt and an angled latch bolt below.

There is a second tell on that same faceplate. Near the top you will find a small set screw, sometimes a pair of them. That screw retains the key cylinder, which threads into the lock case from the outside face. Back it off two turns and the exterior cylinder unscrews counter-clockwise, keys and all. Nothing on a tubular deadbolt behaves that way; its exterior half is held by two long through-bolts reachable only from the inside.

Why the smart deadbolt aisle is closed to you

Nearly every smart deadbolt on the US market is built around one set of holes: a 2-1/8 inch cross bore through the face of the door, a 1 inch edge bore for the bolt, a backset of 2-3/8 or 2-3/4 inches, and a door 1-3/8 to 1-3/4 inches thick. Full replacement locks from Schlage, Yale and Kwikset are cut to that prep, and so is a hidden-in-door design like Level. The retrofit units from August and SwitchBot dodge the prep question but assume a round interior thumbturn, which a mortise case does not necessarily hand them.

A mortise door offers none of that. Its edge already has a pocket routed several inches into it, and residential mortise backsets are commonly 2-1/2 inches, matching neither tubular standard. Boring a fresh 2-1/8 inch hole beside an existing mortise pocket removes material from a stile that has already had a great deal taken out of it, and on a narrow stile there is often not enough timber left to try.

The measurements that decide the outcome

Take these with a tape and write them down. Every route below is ruled in or out by them, and a locksmith will ask for most of them anyway.

On the door edge

• Faceplate length and width, measured plate edge to plate edge rather than bolt to bolt.
• Backset: door edge to the center of the keyway, commonly 2-1/2 inches on a residential mortise case.
• Door thickness at the edge, next to the lock. Old doors are frequently thicker than 1-3/4 inches.

On the outside face

• How far the cylinder stands proud of the escutcheon. Mortise cylinders step in 1/8 inch increments from about 1-1/8 inches upward, and one that projects too far is a security problem as well as an untidy one.
• Center-to-center distance from the keyway to the knob or lever spindle.
• The footprint of the existing escutcheon. Anything new has to cover the old screw holes, or you are filling and painting an entrance door.

On the inside

• The shape of the interior turn piece and how far it stands off the door face.
• The diameter of the rose or plate around it.
• Clearance from the turn piece to the door edge, to any applied moulding, and to the arm of a jamb-mounted closer, which apartment entry doors very often carry.

Route one: a mortise-body smart lock

This replaces the whole case with an electrified mortise lock, motor and all. It is the route hotels and larger apartment buildings take, and the only one that puts a keypad on the outside face of a mortise door without compromise.

What has to match is the pocket: case height, depth and thickness, backset, faceplate size, the spindle hub position relative to the keyway, door thickness, and handing. Pockets cut in the 1920s rarely line up with a modern case, so a locksmith deepens or widens the mortise and drills fresh through-holes for the interior panel. None of that is reversible, which matters if you rent or sit in a co-op with a hardware clause in the bylaws.

Panel-style mortise locks, the tall slab designs imported from the Asian market, need a long uninterrupted run of stile, and on a door with a glazed light or applied moulding the plate fouls on the trim. Measure the flat area you actually have before falling for a photograph. This is also the most expensive route once labor is counted, so establish what a locksmith charges for a job like this before the hardware is bought.

Route two: change the cylinder, keep the case

Because the mortise cylinder is a separate threaded part, it can sometimes be swapped for an electronic one while the case, the bolts and all the visible trim stay where they are. In Europe this is routine, since euro-profile cylinders are one standard shape and electronic versions are sold everywhere. In the US it is a narrower path: mortise cylinders here are threaded barrels driven by a cam at the back, and the electronic versions live in locksmith supply channels rather than on retail shelves.

If you want it priced, pull the cylinder first: loosen the set screw on the faceplate, unscrew the cylinder, measure the threaded barrel, and note the shape of the cam behind it. Those facts plus the case manufacturer are what a supplier needs. The appeal is that the door is untouched and the exterior appearance survives, which on an old building is frequently the entire point.

Route three: a retrofit over the interior turn piece

Retrofit units clamp over the inside turn piece and drive it with a small motor, leaving the exterior cylinder and your existing keys alone. August and SwitchBot are the familiar examples. On a tubular deadbolt with a round thumbturn they are close to a certainty; on a mortise case they are a genuine maybe, and the answer is physical rather than a matter of brand.

Several things get in the way. The turn piece on a mortise case is often a small spade or lever rather than a round knob, so the supplied adapters may find nothing to grip. The retrofit body wants a flat clear area of door around that turn piece, and a mortise interior plate is large and frequently sits close to the door edge. A mortise deadbolt also throws further than a tubular one, which means more work for the motor on every cycle.

There is a test that settles the last of those in ten seconds. Throw the deadbolt by hand using two fingers on the turn piece. If it takes real effort, or you find yourself lifting the door on its hinges to get the bolt home, no battery-driven motor will do that reliably for two winters. Fix the alignment first, or take another route. The measure-first discipline that governs any retrofit fit check applies here with much less margin.

Storm doors, steel doors and anything thicker than 1-3/4 inches

These arrive in the same conversation as mortise doors for the same underlying reason: not enough material in the right place for a standard tubular deadbolt.

Storm and screen doors

An aluminum or vinyl storm door has a stile 1 to 1-1/2 inches wide. There is no 2-1/8 inch bore to be had, and the latch hardware is proprietary, so it gets replaced with that manufacturer’s part or not at all. The smart lock belongs on the main entry door behind it, which has two consequences. The storm door has to be left unlatched for a keypad code to be any use to a visitor, and a closed storm door on a sun-facing entry traps hot air against the main door face. Lock makers publish an operating temperature range, and that cavity is one of the few spots on a house that reaches the top of it.

Hollow steel doors

A steel door is a thin skin over a light core. If it left the factory with lock prep, a tubular smart lock fits it as happily as wood, and the mounting is often better because the prep is reinforced from behind. Cutting fresh prep is a different job: a bi-metal hole saw run slowly with cutting fluid, and an edge that may have nothing solid behind the skin for the latch screws. Commercial hollow metal doors are frequently prepped for a mortise case, or for a cylindrical lock at a 2-3/4 inch backset, so establish which you have before ordering.

Doors over 1-3/4 inches

Most smart deadbolts publish a supported range of 1-3/8 to 1-3/4 inches. Several makers list a thick-door kit with longer mounting posts and a longer tailpiece, generally reaching around 2 to 2-1/4 inches. Measure the door at the lock rather than the frame, and confirm the kit exists while you can still change your mind.

One point to settle separately. If the door carries a metal fire rating label on the hinge edge, cutting new holes can affect that label, and what is permitted varies by jurisdiction and by building. Ask your local building department and the building manager rather than a forum.

When none of the three routes work

Plenty of mortise doors end up here, particularly rented ones where the case may not be touched. An auxiliary rim lock mounted on the interior face needs only a small hole through the door for its cylinder, and some interior keypad bolts need no hole at all, so either adds security without disturbing the mortise. Or the smart lock goes on a different door: a side entrance, a garage-to-house door, a back door with ordinary tubular prep.

If you rent, the lease question comes before the hardware question, and the renter-friendly options and deposit rules are worth reading first. And if the edge of your door turns out to show two small faceplates after all, the ordinary smart lock installation sequence applies and none of this is your problem.

Common questions

Can I just replace a mortise lock with a normal deadbolt?

Physically yes, and locksmiths do it with a mortise conversion kit: a wrap plate covers the old pocket and the new tubular prep is cut through it. It is a downgrade in hardware, it is permanent, and on a co-op or a rental it usually needs written permission first. On a door with any character to it, the conversion stays visible forever.

How do I find out what size mortise cylinder I have?

Take it out and measure it. Loosen the set screw on the edge faceplate, unscrew the cylinder counter-clockwise, then measure the threaded barrel from the collar to the end. Sizes step in 1/8 inch increments from about 1-1/8 inches, and the cam matters as much as the length, so photograph it before it goes back in.

Will an August or a SwitchBot fit my mortise thumbturn?

Sometimes. It depends on the shape of the turn piece, the flat area of door around it, and how hard the bolt is to throw by hand. Turn it with two fingers before you order. Easy means possibly, stiff means no.

Is a mortise lock more secure than a modern deadbolt?

A well-made mortise case in a solid door is strong hardware, and it resists frame spreading in a way a bare tubular bolt does not. The weak point is usually the strike and the jamb, where 3 inch screws into the framing behind the casing change real resistance more than any badge on a box.

The short version

Look at the door edge first: one tall faceplate with two bolts through it means a mortise case, and almost nothing in the smart deadbolt aisle will fit it. Measure the backset, the door thickness, the cylinder projection and the clear space around the interior turn piece before you shop. The three real routes are a mortise-body lock fitted by a locksmith, an electronic cylinder swapped into the case you already own, or a retrofit clamped over the interior turn piece, running from most expensive and most permanent to least of both. If the bolt is stiff to throw by hand, fix that before adding a motor to it. On a storm door, a thin steel door or anything over 1-3/4 inches thick, check the prep and the published thickness range before the box is open.