Replacing or installing a new mortise lock? The key to a smooth process is accurate measuring. Here’s a guide to help you choose the right size lock for your door.
1. Know the Lock Type
There are two main types of mortise locks: deadlocks and sash locks. Deadlocks are common for front doors, while sash locks are typically used on back or interior doors. Deadlocks are easier to measure since they don’t require handle alignment.
2. Measure the Door Thickness
Mortise locks come in different sizes, so it’s essential to measure the thickness and width of your door stile. This ensures the lock fits properly without damaging the door.
3. Consider the Door Material
Mortise locks are only compatible with wooden doors. If your door is made from a different material like steel or fiberglass, consider other lock options.
4. Security Level
If you’re installing the lock on a front door, opt for a 5-lever mortise lock for added security. A 5-lever lock is more difficult to pick and provides better protection than a 3-lever lock.
5. Measure an Existing Lock
If you’re replacing a lock, measure the forend length and width (the part you see on the door’s edge). This ensures a precise fit for the new lock.
How to Measure
For a deadlock, take the following measurements:
• Backset: The distance from the door’s edge to the center of the lock.
• Case Depth & Height: These determine the space the lock will occupy in the door.
• Forend Length & Width: Visible parts on the door’s edge.
The five key dimensions are shown in the diagram below (sashlock shown):
Dimension A: Backset
Backset is measured from the centre of the keyhole to the edge of the lock. Most backsets are commonly measured at either 44mm (for a 64mm (2½”) lock) and 56mm (for a 76mm (3”) lock). Typically a lock adjacent to a glass panel will be 2½” (44mm) and otherwise 3” (56mm), but you should always measure to be sure.
Dimension B: Centre (Pz)
Measured from the centre of the keyhole/ cylinder to the centre of the follower hole (the square hole the handle spindle passes through). This dimension isn’t required for a deadlock as there is no handle.
Dimension C: Case depth
The depth of the lock when routed into the door, measured from edge to edge. Typically in the US case depths are generally 60mm. However, please check before installing, especially if you are replacing an existing lock.
Dimension D: Case height
The height of the lock case, measured from top to bottom.
Dimension E: Forend (faceplate) length
The overall length of the forend/faceplate (the part that is visible once the lock is fitted). The end of the faceplate may be squared or round.
Dimension F: Forend (faceplate) width
The width of the forend/faceplate (the part that is visible once the lock is fitted). The end of the faceplate may be squared or round.
For sash locks, follow the same steps, but also measure the center where the spindle (door handle) passes through. Accurate measuring helps you avoid door damage and ensures a perfect fit for your lock. Follow these steps to save time, money, and ensure proper security.
