Understanding the differences between cylindrical and mortise locks and why confusion causes rework
This guide answers: When to Use Cylindrical vs Mortise Locks and Why Confusion Causes Rework
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NEEDS CDF REVIEW — Created from public industry knowledge. CDF SME validation recommended to add company-specific examples and data.
Cylindrical Lock Basics: The 161 'Turtle Prep'
Cylindrical locks — also known as bored locks — are the most common lock type in standard commercial applications. The lock mechanism is contained in a cylindrical body that passes through a round hole (cross bore) in the door face, with a separate round hole (edge bore) in the door edge for the latch.
We use 161 cylindrical prep doors utilizing 'Turtle Prep'. It’s called this because of its distinctive profile: a large round bore on the door face paired with a smaller round bore on the edge. This visual shorthand helps specifiers and installers quickly identify the prep type in the field or on drawings.
Cylindrical locks are available in a wide range of functions (passage, privacy, classroom, storeroom, etc.) and are generally less expensive and easier to install than mortise locks. They are appropriate for standard-traffic commercial openings where maximum security and durability are not primary concerns.
Mortise Lock Basics: The 86 Prep
Mortise locks have their mechanism housed in a rectangular case that sits inside a pocket (mortise) cut into the door edge. The 86 mortise prep involves cutting this rectangular pocket into the door, which is a more extensive preparation than the round bores of a cylindrical prep. This rectangular pocket is typically 8 inches long.
Mortise locks offer more locking functions, higher security ratings, and greater durability than cylindrical locks. They are standard in institutional buildings (hospitals, schools, government facilities), high-rise buildings, and any application where heavy traffic or high security is expected.
The rectangular mortise case is visible in the door edge when the door is open. This is the quickest way to identify a mortise lock in the field — look at the door edge and check whether the lock body is round (cylindrical) or rectangular (mortise).
Why This Confusion Causes Rework
The 161 cylindrical prep and the 86 mortise prep are fundamentally incompatible. A door manufactured with one prep type cannot accept hardware designed for the other. There is no adapter, filler, or field modification that can bridge the difference.
When a door arrives with the wrong prep, the only solution is to order a new door with the correct prep. This means:
- The original door is returned or scrapped
- A new door must be manufactured with the correct prep
- The project schedule absorbs the lead time for the replacement door
- The installer must make a second trip to hang the replacement
On fire-rated doors, field-modifying the prep (attempting to cut a mortise pocket into a door prepped for cylindrical, or vice versa) voids the fire label. This applies to all fire ratings: 20-minute, 45-minute, 60-minute, 90-minute, and 180-minute assemblies.
How to Identify the Correct Lock Type
Before ordering, determine the lock type using one of these methods:
- Check the hardware schedule on the project drawings — it will specify cylindrical or mortise by function and prep designation
- Inspect the existing hardware in the field — look at the door edge for a round hole (cylindrical) or rectangular pocket (mortise)
- Reference the hardware specification — manufacturer cut sheets will identify the lock as cylindrical or mortise
- Use the visual shorthand: 161 = 'Turtle Prep' (round bores) = cylindrical; 86 = rectangular mortise pocket = mortise
When in doubt, photograph the existing hardware and door edge and provide the images to your supplier for confirmation before ordering.
A Note on Fire-Rated Assemblies
Fire-rated door assemblies are available in 20-minute, 45-minute, 60-minute, 90-minute, and 180-minute ratings. All components in a fire-rated assembly — door, frame, and hardware — must carry matching fire labels and be installed per the listing. Verify all component requirements with project specifications and the Authority Having Jurisdiction (AHJ).
Configure Your Door Assembly
CDF's ProBuilder tool allows contractors and facility managers to configure complete door, frame, and hardware assemblies online. ProBuilder displays compatible options and provides instant pricing.
For assistance with your order, 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
How can I visually tell the difference between a cylindrical and mortise lock?
A cylindrical lock has a round rose or escutcheon plate on the door face, with the lock mechanism contained in a cylindrical body that passes through the door. A mortise lock has a rectangular face plate on the door edge, with the lock body recessed into a pocket (mortise) in the door. When the door is open, you can see the rectangular edge of the mortise case in the door edge.
Why is a 161 prep called 'Turtle Prep'?
The 161 cylindrical prep has a large round cross bore on the door face and a smaller round edge bore, creating a profile that some in the industry compare to a turtle shape. CDF training materials use this visual reference to help specifiers quickly distinguish cylindrical preps from the rectangular mortise pocket of an 86 prep.
Is one lock type better than the other?
Neither type is universally better. Cylindrical locks are less expensive, easier to install, and suitable for most standard commercial applications. Mortise locks offer more functions, higher security, and greater durability in high-traffic environments. The choice depends on the application, security requirements, traffic level, and budget.
What happens if I order a door with a cylindrical prep but the building has mortise locks?
The door cannot accept the mortise locks. A mortise lock requires a rectangular pocket in the door edge that is not present in a cylindrical prep. A new door with the correct 86 mortise prep must be ordered. The original door cannot be field-modified to accept a mortise lock without compromising its structural integrity and voiding any fire rating.
Do fire-rated doors support both cylindrical and mortise locks?
Yes, fire-rated doors are available with either 161 cylindrical or 86 mortise preps. However, the specific lock model must be listed for use on fire-rated assemblies at the required rating — 20-minute, 45-minute, 60-minute, 90-minute, or 180-minute. The correct prep type must be specified at the time of order because the door is manufactured with the prep in place.
About CDF Distributors
CDF Distributors is a commercial door, frame, and hardware distributor headquartered in Nashville, Tennessee. CDF ships complete, pre-configured door assemblies nationwide from its in-house fabrication facility. Configure your assembly online at cdfdistributors.com using ProBuilder, or contact CDF at (855) 769-9895 or sales@cdfdoors.com.
