How to Measure for a Mortice Door Knob

A mortice door knob set has two mortice plates -- one that is screwed onto the edge of the door over the mortice hole and another screwed into the door frame. These plates are placed so that they are flush with each other when the door is closed. This ensures that the door knob mechanism slides smoothly into the door jamb when the door is closed. Accurate measurements are essential; if the plates do not match up correctly on the door edge and the door jamb, the door may not close properly and the door knob may fail to work.

Measuring tape
Level
Shim

Instructions

- 1
  Measure the width of the edge of your open door with a measuring tape. The number of inches will determine the type of mortice plate that will fit your door. The mortice plate you use cannot be too wide, as it will impede the use of the door knob and lock.
- 2
  Measure from the cross bore, or the mortice hole, bored into the side edge of the open door to the floor. This measurement will determine how high the mortice plates are placed on both the door and the door jamb.
- 3
  Check the floor for levelness. If the floor is not level, place a shim under the door frame, place the level on top of the shim and check for levelness again. Use the measuring tape to measure from the shim up to the hole on the door frame. This measurement must equal the measurement from the door mortice hole to the floor in order for the door knob assembly to work correctly.
- 4
  Use the measuring tape to measure the diameter of the cross bore on the open door. The diameter is the measure, in inches, of the cross bore from side to side. The mortice plates must have a diameter at least as big as the core bore diameter.

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