How do I tell if my door lock cylinder is snap-resistant?

Reading the markings on the cylinder you already have, checking how far it stands proud, and deciding whether it needs replacing.

Difficulty: beginner
Time: 15 min
Read: 4 min

Short answer

Look at the face of the cylinder for a kitemark with TS007 stars, a Sold Secure Diamond marking, or SS312. Three stars, Diamond or SS312 Diamond means it has been tested against snapping. No marking at all, plus a cylinder standing more than about 3mm proud of the handle, is the combination worth changing.

A euro cylinder is the barrel your key goes into on a uPVC or composite door. Older ones have a known weak point at the middle, where the fixing screw passes through, and the modern answer is a hardened, sacrificial design tested to a published standard. You can tell which one you have in about a minute without taking anything apart.

What you'll need

A torch
A steel rule or tape measure
Reading glasses — the markings are small (optional)

Why a correctly sized, tested cylinder resists attack

Step by step

Open the door and look at the outside face of the cylinder.The markings are usually stamped or laser-etched around the keyway on the external face, sometimes on the cam bar visible in the door edge. A torch held at a low angle makes faint etching readable.
Look for the TS007 star rating.A British kitemark with one or three stars. Three stars means the cylinder alone has passed the enhanced test. One star means it must be paired with a two-star security handle to reach the same overall level — one star on its own is not the finished job.
Look for Sold Secure Diamond or SS312.A separate scheme run by the Master Locksmiths Association. Sold Secure Diamond, marked SS312, is the other widely accepted mark of a cylinder tested against snapping, and insurers and police schemes treat it as equivalent to three-star.
Treat an unmarked cylinder as untested.Plenty of cylinders are sold described as anti-snap with nothing to back it. If there is no kitemark and no Sold Secure marking, assume it has not been tested, whatever the packaging said.
Measure how far it projects.Hold a rule against the handle plate and see how much cylinder stands beyond it. More than about 3mm gives an attacker something to take hold of. Flush, or as near as the handle allows, is the target — and a cylinder that is simply too long is a very common fault.
Check the handle as well as the cylinder.On a one-star cylinder the security handle is doing half the work. A two-star handle is marked in the same way. A plain decorative handle over a one-star cylinder leaves the pair unrated.
Check every door with a euro cylinder, not just the front.Back doors, side doors, French doors and garage personnel doors usually have the same lock type and are frequently the ones left as the builder fitted them.
Decide whether to change it.An unmarked cylinder, or one projecting well proud of the handle, is worth replacing — a tested cylinder costs little and fits in the same hole. Measure it properly first; a cylinder is measured from the centre fixing screw to each end separately.

Tips

Photograph the markings before you go shopping. Comparing the stamp on the shelf with the stamp on your door removes most of the guesswork.
If you fit a thumbturn cylinder inside for easier escape, keep the door away from glass a hand could reach through, and think about whether everyone in the house can operate it.
Your insurer's wording may name a standard specifically. Matching it exactly matters more than buying the most expensive cylinder in the shop.

Common mistakes

Treating a one-star cylinder as finished — The one-star rating assumes a two-star handle is fitted over it. On its own it has not been tested to the enhanced level and the combination is what the standard describes.
Buying a longer cylinder than the door needs — An over-long cylinder projects past the handle. That projection is the thing every anti-snap design is trying to remove, so it partly undoes the upgrade you paid for.

If it doesn't work

No markings anywhere on the cylinder

Cause: An untested builder's-grade cylinder, or wear has erased the etching — Fix: Assume untested. A tested replacement is inexpensive and is a straightforward swap once you have measured the two halves.

Cylinder standing 5-10mm proud of the handle

Cause: Wrong length fitted at some point, or a handle change — Fix: Measure both halves from the centre screw and fit the correct length, or add a security handle that covers the projection.

Key is stiff and the cylinder is hard to turn

Cause: Usually door or keep alignment rather than the cylinder itself — Fix: Check the door alignment before blaming the lock — a dropped door loads the mechanism. A dry graphite or PTFE lubricant suits cylinders; oil collects grit.

Questions people ask

Is a three-star cylinder worth the money on a back door nobody sees?

Yes, and arguably more so. Doors out of sight of the street are the ones that get attention because whoever is there cannot be seen from anywhere. The cost difference between an unmarked cylinder and a tested one is small.

Does an anti-snap cylinder make my door secure on its own?

No. It removes one specific weakness. The frame fixings, the keeps the multipoint bolts engage into, the glazing beside the lock and whether the door is actually locked with the handle lifted and the key turned all matter as much.

My door has a mortice lock, not a cylinder. Does this apply?

No. Timber doors with a five-lever mortice deadlock are rated under BS3621 instead, shown by a kitemark on the forend in the door edge. Different standard, different failure mode.