How to Replace a uPVC Door Lock: A Step by Step Guide

The euro cylinder is the part the key goes into. Change it if the key is stiff, lost or you want better security. The mechanism is the long strip inside the door edge. Change it if the handle lifts but the door does not lock. If you are not sure which has failed, our uPVC door will not lock guide helps narrow it down.

Open the door and find the small screw in the door edge, level with the cylinder, in the centre of the faceplate. Measure from that screw to the outside end of the cylinder, then from the screw to the inside end. Those two measurements, for example 40 and 45, describe the cylinder size. Measure to the end of the cylinder, not the handle plate.

The cylinder should sit roughly flush with the handle plates. One that sticks out too far is easier to attack, which is why many people fit an anti snap cylinder at this point; see anti snap lock upgrades.

• Open the door and keep it open throughout
• Undo the retaining screw in the door edge and keep it safe
• Insert the key and turn it slightly, about ten degrees, until the cam lines up with the cylinder body
• Pull the cylinder out gently using the key
• Insert the new cylinder with its key, line up the cam and slide it in
• Refit the retaining screw, then test with the door open before closing it

Mechanisms must match on several measurements. The backset is the distance from the door edge to the centre of the handle spindle. The centres are the distance between the spindle and the centre of the cylinder hole. You also need the overall length, the faceplate width and the type and position of the locking points: hooks, rollers or bolts.

Many mechanisms have the maker's name and some measurements stamped on the faceplate. Photograph it before you start. If the old mechanism is no longer made, a compatible replacement must line up with the existing holes in the door and keeps in the frame.

• Open the door and remove the cylinder as above
• Undo the handle screws on the inside and remove both handles
• Remove all the screws along the faceplate in the door edge
• Slide the mechanism out carefully; it may need easing past the seals
• Slide the new mechanism in, refit the faceplate screws, handles and cylinder
• Test the handle and key several times with the door open, then closed

Everything above assumes the door is open. If the mechanism has failed with the door locked, you cannot remove it from the edge, and forcing it will damage the frame. See our guide to a uPVC door that will not open instead.

It is worth getting help if the door has dropped and the new lock will be under strain, if the measurements do not match anything you can find, if the screws are seized, or if the door is locked shut. Fitting a new mechanism in a misaligned door is the most common DIY mistake; it often fails again quickly.

Our uPVC door lock replacement service covers measuring, supplying and fitting, and checking the door alignment at the same time. For an idea of price, see the uPVC door lock replacement cost guide. To book, request a quote or call.