A mortice latch when fitted is concealed within the door except for the forend and the protruding latch. Despite this, the latch should be accurately fitted as it marries up to the lever handles which when fitted, and both work as one.
In most cases the 65mm latch is used, but a 75mm variant is also produced, and used when door knobs are fitted rather than handles. The longer latches are also used when fitted to a ‘Fire Door’ where the door casing rebate is greater than the standard at 25mm.
The mortice latch is usually positioned midway to the door height, so that it corresponds to the ‘lock block’ of the hollow core door. With the modern panelled hollow core doors, it’s often seen that the latch and handles are fitted in line with the portion of the door that represents the middle rail. Technically speaking, this is not correct, as a latch or lock would not normally be fitted at this position, as it would (in the case of a solid panel door) mean morticing out the existing tenon of the door. This would only serve to weaken the door construction, and well as being quite difficult to cut, as it would mean cutting into the end grain of the tenon. For this reason, the middle rail of a door is not actually in the middle of the door as the name suggests, but slightly lower.
Though the hollow core door does not have a joint at this position, the latch should not be position there, as it would not be a consistent with normal practice and look unprofessional.
It is also important that the exact height position of the mortice latch and handles be adhered to, so that if there is more than one door to be fitted, all the handles will be at the same height, and line up with each other.
To fit the latch, begin by measuring down from the top of the door to the centre, and using a square, draw a short horizontal line at this point. This position will also be transferred onto both faces of the door as shown in the first drawing below.
Now measure the distance from the forend of the mortice latch to the centre of the spindle hole as shown in the drawing on the right, and transfer this distance from the edge of the door, onto both horizontal lines on the door face as shown in the second drawing below. This can also be done using the marking gauge if preferred. At this distance on the horizontal lines, define the position by drawing short vertical lines to form a cross, which is the position where the door will be bored out and allow access to the spindle position of the latch.
The spindle is the short square metal bar that is provided with the door handles, and makes the connection between the handles and the mortice latch. Pushing down the door handle causes the spindle to revolve within the mortice latch, so that the spring loaded latch retracts into the body of the mortice latch.
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