Mortice Locks 101 – How to Check for Quality & Identify the Right Replacement

Our 4 easy tips on how to quality check your existing mortice locks, plus advice on choosing the best replacement for your door

Firstly we’d better explain - what is a mortice lock?

Mortice locks (also known as mortise locks) are usually used on external doors and are always operated using a key. Rather than being attached onto the door and door frame, mortice locks are embedded inside a slot (or a "mortice") inside the door itself, with a strike plate or box keep fitted to the door frame. While the installation on these can weaken the door structure, the lock itself is considered to be much stronger and more attack resistant than a bored cylindrical lock or rim lock (Yale lock type).

4 Top Tips – Quality Checking & Replacing Mortice Locks

1) Inspect your current mortice keys for letters & numbers. If they feature letters and/or numbers, they are likely to be only a 2 or 3 lever lock, which aren’t considered to be high security. 5 lever locks are recommended, preferably to BS3621 standards. Insurance companies often require at least 1 external door on homes to be fitted with a BS3621 lock as these have 5 levers, providing a higher level of security. If your home is subject to attack and you don’t have a BS3621 lock, some insurance companies may not provide compensation for any loss. To check whether a lock is certificated to BS 3621, look for any indicative markings - the British Standard kitemark will be visible on the face plate of the lock.

2) Ensure the lock works smoothly from both sides – if not, check for a half shot bolt and/or key turning stiffly or clicking to identify the issue – this is often caused by the key being worn as opposed to the lock being worn, and this can usually be replaced. Please note that in this case it’s not always a good idea to get a copy of the key cut from your existing one, as that may be damaged – instead choose a master locksmith to inspect and create a new key for the lock. Always test the lock in the open position as well – if it works smoothly, it could be jamming against the frame.

3) Check whether the key wobbles in the keyhole, as this can indicate damage of the bolt thrower. Generally speaking, a replacement bolt thrower is an easier and more economical choice than a repair – see your local master locksmith in this case.

4) Does the latchbolt spring out effectively, and does the key have appropriate spring tension? If the answer to either of these is no, your lock may require a spring replacement.


So if you encounter any of the above problems, you might need to choose a replacement mortice lock. Ideally, your local master locksmith can do all of this for you. However, if for any reason you wish to do this yourself, there are a few basic but essential measurements you will need to take of the existing lock in order to ensure you choose the correct replacement mortice lock, otherwise you may find that the lock will not fit into the mortice. This is where we can provide some advice……

Firstly you’ll want to identify what type of mortice lock is currently installed – this may be a dead lock or a sash lock. For these two lock types, most measurements are the same, however it is important to remember that with sashlocks, you will need to take note of the “centres”. The centres are measured from the centre of the follower (the square spindle hole), and the centre of the rounded part of the keyhole – take a look at our helpful diagrams to show you where to measure from and to, on both sash-locks and dead-locks.




Deadlocks have a single deadbolt (usually operated by the key) and no latch.

A     Case Depth
B     Backset
C     Case Height
D     Forend Length
E     Forend Width

Sashlocks have a deadbolt (usually operated by a key) and a latch (usually operated by door furniture)

A     Case Depth
B     Backset
C     Case Height
D     Centres
E     Forend Length
F     Forend Width

If you found this information useful, you may wish to check out some of our other helpful "how to measure" guides by clicking HERE.

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