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How to Measure - Install Half Mortise Locks

Half Mortise Locks for furniture and cabinets: how to measure and install

What you need to know about selecting and replacing half mortise locks

Half Mortise Locks are a traditional lock style used on 18th, 19th and 20th century furniture. They offer a semi-concealed appearance. The keyhole is all that is seen on furniture exterior. A simple flush plate is seen on interiors of furniture.  

There are two types of half mortise locks:

How to Measure a Half Mortise Lock

Half Mortise Drawer Locks

Half Mortise Drawer Locks are fitted into top portion of drawer interior. Lock is only visible from inside of drawer.
  • When locked, bolt protrudes up into notched area in rail above drawer. 
  • A strike is rarely used with this application.
  • Keyhole is all that is visible on drawer front.
  • Use with keyhole cover for a finished appearance. 

Half Mortise Door Locks

  • Half Mortise Door Locks are handed, meaning specific right-hand or left-hand types are required. When looking for door locks be sure lock is handed according to your needs. Please note: some half mortise locks are not available in both left and right options. 
  • Doors hinged right - bolt shoots left.
  • Doors hinged left - bolt shoots right.
Box Lock Terminology  

Half Mortise Box Locks

Box locks are specifically designed for boxes and chests with lift lids. They can only be used for this type application.

These box locks require a strike plate with hooks. When lid is closed, strike-hooks drop down into lock and are secured by locking mechanism. 

If Replacing a Lock: the backset measurement of the lock and the backset measurement on the box itself must match. If the box size differs from what you need, but the backset matches, reworking the mortise may make the lock workable.


Measuring Half Mortise Locks:

Before looking for a replacement it will be necessary to measure your lock:

  • Diagram above displays lock measurements.
  • The location of the keyhole in the lock must match the location of the keyhole in the furniture -there are no exceptions to this rule!
  • This measurement is the backset, also referred to as "distance to pin". It is the most important measurement. 
  • This measurement must match precisely for the lock to lineup with the keyhole in the furniture. As the key, when inserted through your furniture keyhole, must line up with the keyhole in the lock.
  • Basically, the backset is the distance between the selvedge/bolt edge of the lock and the keyhole pin. 
  • Once the backset measurement is determined you can look at your various options.
  • If potential substitutes have different lock-body or lock-plate measurements cabinet work may be necessary to fit the lock.

Replacing Half Mortise Locks:

Many types and sizes of locks have been made over the years. Sadly, few are in production today. As a result, replacing a lock may require an adjustment to the cutout, in the furniture, that houses the lock.

Location of keyhole in lock must match position of keyhole in furniture.

This measurement is the backset. Pictures above show this.

Check by:

  • Measure Keyhole Location in Lock: distance between selvedge/bolt-edge - where bolt shoots through, to lock pin. This measurement is known as the backset.
  • Measure Keyhole Cutout Location in Furniture: distance between drawer/door/box edge (where lock will be placed) to estimated location of lock pin in furniture keyhole.

Installing Half Mortise Locks

Half Mortise Lock Installed
Half Mortise Lock Install
  • Adjust position to allow for location of keyhole as it is often off center in lock
  • Mark location of lock on drawer or door interior
  • Hold lock at desired location against drawer or door interior
  • Selvedge edge should be level with top of drawer or edge of door
  • Trace outline of lock on drawer or door interior
  • Trace outline of selvedge on drawer top or door edge
  • Inspect outline for accuracy , straighten lines where necessary
  • Again holding lock in position, trace lock-body
  • Cut shallow mortise for lock plate and selvedge with router or chisel
  • Selvedge and lock-plate should be flush with the interior wood surface once installed
  • Cut lock-body-mortise. Okay if off slightly as lock-plate will cover lock-body once installed. Be sure there is ample room for mounting screws to attach lock
  • Use of a strike is rarely necessary on drawers and doors. Traditionally a simple mortise was cut in the wood to accept the lock bolt.
  • However a strike plate with hooks is necessary for boxes and chests with lift lids

  • Keyhole Placement

  • Measure distance from top of lock-plate to center of key pin
  • Measure distance from side of lock-plate to center of key pin
  • With these two measurements locate keyhole on furniture front, make sure key pin is aligned with keyhole location
  • Drill small pilot hole at key pin location, hold lock in position and check to make sure hole lines up with key pin
  • Drill hole larger to accommodate key barrel
  • For a finished appearance: mount an  escutcheon/keyhole plate, on furniture front.

  • Box Locks
  • Follow above directions
  • Align strike on underside of lid
  • Trace outline of strike in position
  • Check strike alignment and make adjustments if necessary. Cut notches for strike hooks
  • Screw into place
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