Mortise backset is a crucial measurement for door locks, specifically those housed within a mortise pocket cut into the edge of a door. It is defined as the distance measured from the edge of the door (where the lock faceplate sits) to the center of the keyhole or the spindle that operates the handle.
Understanding Mortise Backset Measurement
As referenced, backset is measured from the centre of the keyhole to the edge of the lock. This measurement dictates how far the keyhole, cylinder, and door handle will be positioned inward from the edge of the door.
Imagine looking at the edge of a door where the lock is installed. The backset is the distance from this edge surface to the very middle point of where the key turns or the handle post sits.
Common Mortise Backset Sizes
Mortise locks are available with various backset measurements to suit different door preparations and aesthetic preferences. The reference highlights common sizes:
- 44mm (or 2½ inches): This is typically associated with smaller lock cases, often described as 64mm (2½”) locks.
- 56mm (or 3 inches): This size is commonly found on larger lock cases, such as 76mm (3”) locks.
| Backset Size | Associated Lock Case Size | Typical Use Case (Reference) |
|---|---|---|
| 44mm (2½") | 64mm (2½") | Adjacent to a glass panel |
| 56mm (3") | 76mm (3") | Standard applications (otherwise) |
Note: While these are common, it is essential to always measure to be sure when replacing a lock or specifying hardware.
Why Backset Matters
Selecting the correct backset is vital for several reasons:
- Compatibility: The lock's backset must match the preparation (the cuts and holes) already made in the door. Installing a lock with the wrong backset is impossible without modifying the door significantly.
- Handle Placement: The backset determines the position of the door handle on the face of the door, affecting the overall appearance and feel.
- Clearance: As mentioned in the reference, a smaller backset (like 44mm) positions the handle closer to the door edge, which is useful for doors with narrow stiles or those with glass panels that limit space for hardware. A larger backset (like 56mm) places the handle further in, providing more clearance from the door edge.
Choosing the right backset ensures that the lock fits correctly, functions properly, and the door hardware is positioned appropriately for both aesthetics and usability.
