Replacing a euro cylinder yourself is a 10-minute job — but only if you've bought the correct size. The single most common DIY mistake is buying a cylinder that's too long, ending up with brass protruding from the door faceplate. That protrusion is exactly what lock snappers exploit. An over-long cylinder turns a security upgrade into a security downgrade. This guide explains how to measure correctly in under 5 minutes.
Why size matters
Euro cylinders sit through your door with one half visible outside and one half visible inside. The handle escutcheon (the decorative plate around the handle) should sit flush against the cylinder face. If the cylinder is the right size:
- The cylinder protrudes 0-3 mm from the escutcheon on each side
- The cam screw can be tightened firmly without slipping
- The cylinder is too short to grip with mole grips for snapping
If the cylinder is over-long (5+ mm protruding):
- A burglar can grip the protruding section with mole grips
- Lock snapping works in under 30 seconds
- Your security upgrade is worthless
The measurement procedure
Step 1: Open the door fully
You need access to both sides of the cylinder. With the door open and stable, you can measure each side without obstruction.
Step 2: Find the cam screw
On the metal faceplate on the edge of your door (between the latch and the deadbolt), find a single long screw — typically 60-100 mm long. This is the cam screw. Its centre point aligns with the centre of the cam inside the cylinder.
Step 3: Measure the two halves
Using a ruler or tape measure, measure from the centre of the cam screw outwards to each face of the cylinder:
- External face (the side facing outside the house): measure 1
- Internal face (the side facing inside): measure 2
Round each measurement to the nearest 5 mm. Most euro cylinders come in 5 mm increments (30, 35, 40, 45, 50, 55, etc.).
Step 4: Write the format
Euro cylinder sizes are written as External/Internal — for example, 40/50. The first number is the external side (the side facing outside the house). The second is the internal side. Common UK sizes:
- 35/35 (70 mm total) — older slim doors
- 40/40 (80 mm total) — standard residential UPVC
- 40/50 (90 mm total) — common when there's a thumb-turn on the inside
- 45/55 (100 mm total) — composite doors with thicker frames
Common mistakes
Measuring from face-to-face instead of from the cam screw
The cam screw is the centre point of the cylinder — not the geometric middle of the brass body. Measure from the cam screw outwards, not edge to edge.
Buying "one size up to be safe"
The opposite of safe — over-long cylinders are snappable. Match the dimensions exactly.
Confusing external and internal
If you have a thumb-turn on the inside, that side is "internal" — usually the longer side. Standard cylinders (key both sides) often have the longer side internal too. Confirm by orientation when fitting.
Buying a "high-security" cylinder that's too long
Even a 3-star TS007 anti-snap cylinder is snappable if it protrudes excessively. The "sacrificial" front section is designed for a small attack length — not 10 mm of exposed brass.
What to do if your existing cylinder is too long
If you're upgrading because the previous cylinder was already over-sized, don't replicate the mistake:
- Measure the door thickness directly: open the door, measure the door panel thickness between the two faces
- Add 5-6 mm for the handle escutcheons (2-3 mm each side)
- The total cylinder length should equal this measurement
- Decide on the offset (whether external or internal half is longer) based on whether you have a thumb-turn
If in doubt, ask the locksmith
A Cornwall locksmith fitting a replacement cylinder will measure your door themselves and bring the correctly-sized cylinder. Cost: £80-£180 fitted for a standard or TS007 3-star cylinder. The saving from DIY (~£40-£80) is small compared to the risk of buying the wrong size and either redoing the job or weakening your security. Worth the locksmith fee on a job that matters.
Need a euro cylinder replaced in Cornwall by a locksmith who'll measure correctly? Submit your postcode for an itemised quote — TS007 3-star or Sold Secure Diamond available.
Frequently asked questions
How do I measure a euro cylinder?
Measure from the centre of the cam screw (on the door edge faceplate) outwards to each face of the cylinder. Note both numbers in millimetres. Format: External/Internal — e.g., 40/50. Round to the nearest 5 mm. Common UK sizes: 35/35, 40/40, 40/50, 45/55.
What happens if I buy the wrong-size cylinder?
If too short: the cylinder won't sit flush; the cam screw won't engage properly; the door won't lock reliably. If too long: brass protrudes from the escutcheon, giving burglars an easy grip for lock snapping — defeating any security rating the cylinder has. Match dimensions exactly.
Why does cylinder length affect security?
An over-long cylinder protrudes from the door faceplate, allowing a burglar to grip the protruding section with mole grips and snap the cylinder in under 30 seconds. Even a TS007 3-star anti-snap cylinder is snappable if it protrudes excessively — the sacrificial front section is designed for short protrusion only.
How long are standard UK euro cylinders?
Common sizes (External/Internal in mm): 35/35 (70mm total, older slim doors), 40/40 (80mm, standard residential UPVC), 40/50 (90mm, with thumb-turn inside), 45/55 (100mm, composite doors with thicker frames). Always measure your specific door — sizes vary by manufacturer.
Should I just call a locksmith?
For most people, yes — Cornwall locksmiths charge £80-£180 fitted for a euro cylinder replacement, and they'll measure correctly and bring the right size. The £40-£80 DIY saving is small compared to the risk of buying the wrong size and either redoing the job or weakening your security.