What is the difference between a euro, oval and rim cylinder?
How to recognise each of the three lock cylinders found on British doors, which door each belongs to, and the internal options — key both sides or thumbturn — that matter for escape.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 3 min
Short answer
Euro: a circle with a rectangular tail, held by one long screw, fitted to almost every uPVC and composite door. Oval: a plain flattened oval, mostly on older aluminium doors. Rim: a round barrel with a flat connecting bar, driving a nightlatch on the inside face of a timber door. They are not interchangeable without changing the door.
Cylinders are the part of a lock that the key goes into, and almost the only part a householder ever replaces. Knowing which of the three you have takes one look at the shape and saves buying the wrong thing, and the internal options are worth a moment's thought because one of them is a fire-safety decision.
Step by step
- Look at the shape end-on.Open the door and look at the cylinder from the edge. A circle with a narrow rectangular tail below it is a euro profile. A smooth flattened oval with no waist is an oval profile. A plain circle with two fixing lugs, with a long flat bar out of the back, is a rim cylinder.
- Confirm by the fixing.A euro or oval cylinder is held by one long machine screw entering through the door edge, through the lock, into the cylinder's central hole. A rim cylinder is held by two long screws through the door into a backplate on the inside face.
- Match it to the door.Euro is the standard on uPVC, composite and modern aluminium doors with multipoint locks. Oval turns up on older aluminium and on commercial doors. Rim goes with a nightlatch — the surface-mounted case on the inside of a timber front door.
- Understand how a euro cylinder is sized.It is measured as two halves either side of the central fixing screw, always quoted outside first — a 35/45 is 35mm outside and 45mm inside, an 80mm cylinder overall. The two halves are frequently different, and getting them the wrong way round leaves the cylinder standing proud outside.
- Decide what goes on the inside.Key both sides means nobody can open the door from inside without a key, which stops someone reaching through broken glass but also stops you getting out in a hurry. A thumbturn inside means you can always get out, which is the safer choice unless the turn can be reached.
- Check whether the turn can be reached.If there is glass in or beside the door, or a letterplate within arm's length of the cylinder, a thumbturn is reachable and should not be fitted. In that situation the answer is laminated glazing, a letterplate cowl, or keeping a key on a hook out of reach but always in the same place.
- Look at the standard marking before buying.Cylinders carry markings for the standard they meet and, on the better ones, a star rating. On a front door this is the single most cost-effective security upgrade there is, and it is a five-minute swap.
Alternatives
- Euro half cylinder: A single-sided euro cylinder, used where a lock only ever needs operating from one side — a garage side door or a gate. Not appropriate on a habitable room's exit door.
- Euro thumbturn cylinder: Key outside, turn inside. The default recommendation on a solid front door with no reachable glazing, because escape does not depend on finding a key.
- Keyed-alike set: Several cylinders supplied on one key. Available in euro, oval and rim, and worth ordering as a set when more than one lock is being replaced.
Questions people ask
Can I put a euro cylinder in an oval hole?
No. The cut-out in the lock case is shaped to the profile, and the profiles are genuinely different shapes. Changing from one to the other means changing the lock.
Does a longer cylinder make a door more secure?
The opposite. A cylinder that stands proud of the handle plate gives an attacker something to grip. It should finish flush with, or very slightly inside, the outside handle plate.
Why does my key work from outside but not inside?
On a key-both-sides cylinder, a key left in the inside is enough to block the outside key on many designs. Anti-snap and 'both sides at once' cylinders behave differently, so check which you have.