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Yes, you can replace a mortise lock with a cylindrical lock, but it is a door-prep job, not a hardware swap. The door edge has a large rectangular mortise pocket, while a cylindrical lock needs a 2-1/8 in cross bore and a 1 in edge bore. You fill or cover the pocket (a conversion or filler plate covers the cut-out), bore the new holes, and fit the latch. Do not do this on a labeled fire door.

When does converting from mortise to cylindrical make sense?

Convert when the mortise lock body is obsolete or unrepairable, or when you need a cylindrical-only product, such as a particular keypad or proximity lock. It also makes sense when you are refinishing or replacing the door anyway, so patching the pocket is part of the job. If a current mortise lock fits the existing prep, replacing like for like is usually simpler.

What door prep does a cylindrical lock need?

Schlage's cylindrical lock instructions specify a 2-1/8 in cross bore through the door face and a 1 in edge bore, with a 2-3/8 in or 2-3/4 in backset. Always use the template that ships with the lockset, and confirm the door thickness and hand before drilling. A mortise lock uses a much larger rectangular pocket cut into the edge, so the two preps cannot share the same opening.

Item	Mortise lock prep	Cylindrical lock prep
Door edge	Deep rectangular pocket and faceplate cut-out	1 in round edge bore plus a shallow latch face cut
Door face	Cylinder and spindle holes sized to the lock	2-1/8 in cross bore
Backset	Set by the lock case	2-3/8 in or 2-3/4 in

What are conversion plates and wrap-around plates?

A conversion (filler) plate is a steel plate that covers the old mortise cut-out on the door edge and gives the new latch a prepared surface. A wrap-around plate wraps the door face and edge around the cylindrical bores and is made for door repair and reinforcement. These are made by door-hardware accessory companies. Examples, per the product listings:

• Don-Jo CV-86: an 8 in x 1-1/4 in primed steel filler plate that converts a mortise lock "86" cut-out to accept a "161" cylindrical latch prep.
• Don-Jo 81 CW and 61 CW: wrap-around plates for cylindrical locks on 1-3/4 in doors with a 2-1/8 in hole, 4 in x 4-1/2 in, in 2-3/4 in (81 CW) and 2-3/8 in (61 CW) backsets.
• Don-Jo 943-S-CW: a 4-1/4 in x 9 in plate with a 2-1/8 in double bore for cylindrical locks, 2-3/4 in backset.

Plate fit depends on your existing cut-out and backset, so match the model to the opening before ordering. Confirm the dimensions and cut-out on the manufacturer's specification sheet; a plate listed for one cut-out will not fit another. We carry cylindrical lever locks to pair with a plate.

How do you convert a mortise prep to a cylindrical lock?

1. Remove the mortise lock. Take off the trim and spindle, remove the cylinder, unscrew the edge faceplate, and slide out the case. Note the door thickness, hand and backset.
2. Inspect the door. Check the pocket and edge for cracks or soft spots. A damaged edge must be repaired before any plate goes on.
3. Choose the plate. Match a conversion or wrap-around plate to your existing cut-out, door thickness and the new lock's backset.
4. Fill or cover the pocket. Install the plate to the manufacturer's instructions. On a wood door, a solid wood fill or structural epoxy can be sanded flush where the plate does not cover the pocket.
5. Bore the new holes. Use the lockset's template: 2-1/8 in cross bore and 1 in edge bore at the 2-3/8 in or 2-3/4 in backset. Drill square and from both faces to avoid tear-out.
6. Install the latch and lockset. Follow the lock's installation sheet for the chassis, spindle, trim and screws.
7. Reposition the strike. The old mortise strike location will not line up. Prepare the new strike location per the lock's template, and fill the old strike area.
8. Test. Confirm the latch retracts smoothly, the door closes and latches without force, and the key operates. If the latch drags, re-check strike alignment before anything else.

When should you not convert?

• Labeled fire doors: NFPA 80 limits what can be cut in the field. Mortising or re-boring a labeled fire door for a different lock is generally not allowed without approval from the listing laboratory, and may void the label. Check with the AHJ before touching it.
• Narrow stiles or glass near the lock: there may not be enough material for the 2-1/8 in bore.
• Historic or stained doors: plates and patches show.
• High-use entrances: compare ANSI/BHMA grades, not lock style. Both mortise (A156.13) and cylindrical locks are graded, so choose Grade 1 for heavy traffic.

What are the alternatives to converting?

Replace the old lock with a current mortise lock that fits the existing prep, or change only the cylinder. See the mortise cylinder sizes and cams guide. Many electronic functions are available as mortise trim, so you may not need a cylindrical lock for access control.

FAQ

Can I put a cylindrical lock in a mortise door?

Yes, with a conversion or filler plate that covers the mortise cut-out and new 2-1/8 in and 1 in bores for the cylindrical lock, on a non-fire-rated door. The stile needs enough material for the bores.

Is a mortise lock stronger than a cylindrical lock?

Mortise locks are heavier-duty cases, but strength is decided by grade, not type. Compare the ANSI/BHMA grade on the specific products.

What backset do I need?

Cylindrical locks use 2-3/8 in or 2-3/4 in. Measure from the door edge to the center of the cross bore, or follow the template.

Can I convert a fire-rated door?

Generally no, not in the field. NFPA 80 limits field preparation on labeled fire doors, so check with the AHJ and the door manufacturer.

Official sources

Door prep dimensions are from the Schlage door prep checklist and Schlage cylindrical lock installation instructions at commercial.schlage.com. Plate dimensions are from Don-Jo listings at Trudoor and Quality Door; confirm on the manufacturer's sheet.

Next step

Shop cylindrical lever locks and mortise locks. Orders before 3:00 p.m. ET ship the same business day after card approval; ground shipping is free on $450 and up. For a quote, use get a quote or call 718-814-6300.