Single-cylinder deadbolt (ANSI/BHMA A156.36)
A separate round lock above the handle: key outside, thumb-turn inside, 1 in bolt throw. It is the single most retrofit-friendly lock in the world because the thumb-turn is accessible, standardised, and the door is already drilled.
Also called: Deadbolt · Auxiliary lock · Thumbturn deadbolt
Anatomy
- Cylinder on the outside face
- Thumb-turn on the inside face — the part a retrofit motor grabs
- Bolt with 1 in (25 mm) minimum throw
- Strike and reinforcement plate in the frame
Parameters to measure
Measure in this order: Interior thumb-turn / turn-piece size and profile → Backset (Dornmaß) → Door thickness → Deadbolt throw
| Parameter | Typical values | Tolerance that still fits | How to measure |
|---|---|---|---|
| Backset (Dornmaß) | 2-3/4 in (70 mm) standard; 2-3/8 in (60 mm) also sold | Both are supported by most smart locks | Door edge to the centre of the bore. |
| Cross bore diameter | 2-1/8 in (54 mm) | — | Across the round hole. |
| Deadbolt throw | 1 in (25.4 mm) all grades | — | Throw the bolt fully and measure past the faceplate. |
| Interior thumb-turn / turn-piece size and profile | Round knob, T-bar or coin-slot, 20–30 mm across | This is the dimension adapter kits are built around | Measure across the widest part and note the shape. |
| Door thickness | 1-3/8 to 2 in (35–51 mm) | — | At the bore. |
Retrofit architectures that work
- Surface motor on a deadbolt turn-piece — An adhesive or screw-fixed motor sits on the door face and rotates the existing deadbolt turn-piece. Nothing is drilled. This is the standard approach for North American single-cylinder deadbolts and for many Asian rim deadbolts.
- Motor replacing the interior handle — The interior lever or knob is replaced by a motor unit that drives the 8 mm spindle directly. The outside keeps the original key cylinder and handle. This is the cleanest fit for mortise locks where the bolt is thrown by the key and the handle only retracts the latch.
Adhesive motor on the thumb-turn is a 5-minute install and needs no tools beyond a screwdriver. The engineering work is all in the turn-piece adapter library and in adhesive that survives a hot door in summer.
Related standards
Reference devices
Recorded as engineering examples, not endorsements.
- SwitchBot Lock Pro — Surface motor on a deadbolt turn-piece ✓ verified
- Aqara Smart Lock U200 — Motor replacing the interior handle ✓ verified
- August Smart Lock (Pro / Wi-Fi) — Surface motor on a deadbolt turn-piece ✓ verified
Questions
Why do US smart locks not fit European doors?
They are built around a round bore with a thumb-turn. A euro cylinder door has neither, so the motor has nothing to grab and no bore to sit in.
What happens when the battery dies?
The outside key still works — that is the point of keeping a single-cylinder deadbolt rather than a keyless one.