The Yale 5 lever deadlock offers a high level of security, and is suitable for use on external wooden doors when used in conjunction with a maximum security nightlatch.
If you are replacing an existing mortice lock, you can find out what size you need without removing your old lock. Simply measure from the edge of the door to the centre of the keyhole. If the measurement is 45mm (1 3/4 ), you need a 64, 65 or 67mm (2 1/2 ) mortice lock. If the measurement is 57mm (2 1/4), you need a 76, 78 or 80 mm (3 ) mortice lock.