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How to replace a euro cylinder without locking yourself out

Updated: October 2026

The door is open, the screwdriver is in your hand, and the old barrel is still in the lock. How to replace a euro cylinder is a short job when you measure both sides, put the knob on the inside, and prove the new one before you shut the door. The [knob cylinder comparison](/hub/hot-water/knob-cylinder) is the table of four listings. This page is the swap, from public UK fitting guides and those amazon.co.uk pages on 19 October 2026. A qualifying purchase through our links can earn a commission at no extra cost to you. Whether a thumbturn is the right function sits on the [buying guide](/hub/hot-water/knob-cylinder/sub/knob-cylinder-buying-guide).

The door is open, the screwdriver is in your hand, and the old barrel is still in the lock.

Our top picks

GANZTECH GANZTECH 30/40T chrome

Best overall

GANZTECH GANZTECH 30/40T chrome
8.6/10about £10.99
View offer
Valorada Valorada 55/45T thumb turn

Rank 2

Valorada Valorada 55/45T thumb turn
5.5/10about £13.99
View offer
BGS BGS 85290 rotary knob 45/45

Best value

BGS BGS 85290 rotary knob 45/45
8.3/10about £6.55
View offer
On this page
  1. 1.Tools and the open door
  2. 2.Two measurements, knob excluded
  3. 3.Out with the old, in with the knob inside
  4. 4.When this is no longer a cylinder job

Tools and the open door

You need a manual screwdriver that matches the fixing screw, a tape, the key that already works, and the replacement in your hand before the old cylinder leaves the door. Checkatrade separates the cylinder from the multipoint gearbox. If the hooks will not move when the key turns, a new barrel will not repair that gearbox. Keep the door open. Closing it to see if it works is how people lock themselves out with a cam that is not lined up. Remove a handle only if it covers the fixing screw. Do not use an impact driver. A Winkhaus and RAM leaflet sets a 5 Nm maximum and bars an impact driver. The BGS listing supplies an 85 mm screw and a 35 mm screw. Use the one that matches the old screw.

Two measurements, knob excluded

Measure from the centre of the fixing screw to the outside face, then from the same centre to the inside face. Leave the knob off the tape. Home Secure Shop's guide uses that method and works in 5 mm steps. Seventy millimetres is only the total. A 35/35 and a 30/40 can both add up to 70 mm, and they do not put the screw in the same place. Prefer the two lengths from the screw, and record which side is the turn. Sellers do not all print the T on the thumbturn side.
GANZTECH GANZTECH 30/40T chrome

Testsieger

GANZTECH GANZTECH 30/40T chrome
8.6/10about £10.99
View offer

Out with the old, in with the knob inside

With the door open, unscrew the single fixing screw on the edge. Insert the key and turn it slightly so the cam lines up with the body and the cylinder can slide out. If it will not move, ease the key a few degrees either way. The knob goes on the inside. ERA's sheet says the outside face should finish flush or within 3 mm. West Yorkshire Police say not to leave a euro cylinder proud of the handle. Fit the new screw by hand until it is seated, then test with the door still open. Only then close the door, with the key in your hand. A letterbox that can reach the new knob is a reason to reconsider the function on thumbturn or key both sides.

When this is no longer a cylinder job

Stop if the key will not turn in the old cylinder. Stop if the cylinder will not slide out after the cam is aligned, or if metal is burred around the hole. Stop if the hooks on the door edge do not move when the key turns cleanly. A new knob cylinder will not repair that strip. A fire door, or a final exit in a house in multiple occupation that you do not manage, is not a Saturday experiment. West Yorkshire Police point householders who want the cylinder specified to locksmiths.co.uk. Which of the four listings matches a measured split is on four thumbturn listings.

Common questions

Where is the fixing screw?
Look on the edge of the open door, level with the cylinder, for a single screw into the side of the barrel. It is not one of the handle screws on the face. Remove that screw, line the cam up with a small turn of the key or the knob, and the cylinder should slide out. The BGS listing includes an 85 mm screw and a 35 mm screw. Match the old screw.
How do I measure a thumbturn?
Measure from the centre of the fixing screw to each end of the cylinder body, and keep the knob out of the measurement. Record the outside and the inside separately, in 5 mm steps. The letter T on a title is not a reliable mark of which side is the turn, because sellers do not all use it the same way. A 30/40 and a 35/35 can both look like 70 mm and still miss the screw.
What if the cylinder will not come out?
The cam is usually still turned, so the cylinder is hooked inside the lock. Turn the key or the thumbturn a few degrees until the cam sits flush with the body, then slide. If it remains stuck, do not hammer it and do not keep the door shut while you try. A burred hole or a seized gearbox is a locksmith job. Put nothing back half-inserted and then close the door.
Which way does the knob face?
The knob faces the room. The key faces the street or the landing. On an offset cylinder the longer side is often the inside, but you confirm that from your two measurements, not from a habit. Fit it the wrong way round and the turn is outside, where anyone can use it, or the screw misses the cam. Test both directions with the door open before you rely on it.
When do I stop and call a locksmith?
Stop when the key will not turn, the cylinder will not slide, the hooks do not move, or the door is a fire door or an HMO exit you do not manage. Also stop if you have already removed the cylinder and the replacement does not match, rather than shutting the door on an untested cam. locksmiths.co.uk is the directory West Yorkshire Police give for a specified cylinder. A new barrel does not repair a gearbox.

Related pages

Knob cylinder and TS 007

Knob cylinder

Best thumbturn euro cylinder for the size you measured

Thumbturn euro cylinder: the checks before you pay

Thumbturn or key both sides: which cylinder to order

Sources

Public UK sources named below. amazon.co.uk prices, stars and review counts on these four listings were read on 19 October 2026. Page updated 21 October 2026.

  1. West Yorkshire Police euro cylinder advice (retrieved 19 October 2026)
  2. Secured by Design: locks and associated hardware (retrieved 19 October 2026)
  3. DHF thief-resistant lock assemblies (retrieved 19 October 2026)
  4. DHF TS 007 star ratings (retrieved 19 October 2026)
  5. BSI BS 8621:2017+A1:2024 (retrieved 19 October 2026)
  6. Cheshire Fire and Rescue: houses in multiple occupation (retrieved 19 October 2026)
  7. ERA fitting instructions (retrieved 19 October 2026)
  8. Checkatrade: changing a uPVC door lock (retrieved 19 October 2026)
  9. amazon.co.uk listings B0D5HY325L, B0CLV6BMNX, B0GS5VFV7S and B0H14R6SPH (retrieved 19 October 2026)