5-lever mortice deadlock (BS 3621)
A lever lock keyed from both sides, with no cylinder at all: the key moves the levers directly through a keyhole in the faceplate. BS 3621 certification requires at least five levers, hardplates and a 20 mm bolt throw. Retrofit is hard here — there is no cylinder and no spindle to drive.
Also called: 5 lever deadlock · Chubb lock · British Standard deadlock
Anatomy
- Lever stack — the gate pattern that gives the key its differs
- Deadbolt only, no spring latch
- Faceplate with a keyhole on each side
- Kitemark stamp on the faceplate when certified
Parameters to measure
Measure in this order: Lever count → Lock case width × height × depth → Backset (Dornmaß) → Deadbolt throw
| Parameter | Typical values | Tolerance that still fits | How to measure |
|---|---|---|---|
| Lock case width × height × depth | Case sizes commonly 76 mm or 64 mm (3 in / 2.5 in nominal) | — | Measure the case width across the door edge. |
| Backset (Dornmaß) | 44 mm (76 mm case) or 57 mm, varies by maker | — | Door edge to the centre of the keyhole. |
| Lever count | 5 for BS 3621; 2 or 3 on older and interior locks | — | Count through the keyhole or from the case edge. |
| Deadbolt throw | 20 mm minimum for BS 3621 | — | Key the bolt fully out and measure how far it projects past the faceplate. |
Retrofit architectures that work
There is no clean retrofit path for this family. See the note below.
There is no clean retrofit for a lever lock. The honest options are: (1) replace the case with a euro-cylinder mortise deadlock, which needs a new door prep and, on a fire door, a certified doorset; (2) keep the lever lock and add a separate smart deadbolt above or below; (3) leave it alone — a BS 3621 lever lock is already a good lock. Never design a retrofit that leaves the door unable to be opened by hand from inside.
Related standards
Questions
How do I know if my lock is really BS 3621?
Look for the Kitemark and the words BS 3621 stamped on the faceplate — the strip visible on the door edge when the door is open. Some 5-lever locks are not certified, and a 3-lever lock can never be.
What is the difference between BS 3621 and BS 8621?
Same attack resistance. BS 3621 needs a key on both sides; BS 8621 has a thumb-turn or push-pad inside for keyless escape. BS 10621 locks from outside only.