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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 select Half Mortise Locks for drawers, doors and boxes

Drawer and Door Locks


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. 
 

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.

 

 

Half Mortise Box Locks: important facts necessary in installing locks
 

 

 

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.
  • Please view diagram to determine how to measure your lock.
  • The backset measurement, is the most important measurement, must match precisely
  • 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:

Keyhole location measurement is critical.  Replacement lock-keyhole-location must match location of keyhole in furniture.

To measure: easiest to measure old lock, if you have it:

  • Lock Keyhole Location: distance between selvedge-edge - where bolt shoots through, to lock pin. This measurement is known as the backset.
  • Furniture Keyhole Cutout Location: distance between furniture-edge to estimated location of lock pin in furniture keyhole.

Installing Half Mortise Locks

Determine desired location of lock

  • 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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