How to Change the Lock on a uPVC Door: Barrel, Handle or Mechanism

Every euro cylinder measurement starts from the centre of the fixing screw hole, which lines up with the middle of the cam.

1. Measure outward. Take the distance from that centre point to the outside face, then from the centre point to the inside face. Write the two figures down, outside first.
2. Round it. Tradelocks says to round to the nearest 5mm, so 43mm becomes 45 and 42mm becomes 40, because cylinders are only made in 5mm steps.
3. Mark the sides. Note which side is which before the old cylinder comes out. An offset cylinder such as 40/50, fitted backwards, puts the long side on the outside.

If nothing has been taken apart yet, you can measure in place: go from the centre of the keyhole to each face of the door with the cylinder still fitted, then compare those figures with the old cylinder once it is out.

The best-known advice on this subject says to measure the barrel you have removed, and the suppliers who sell the replacements warn against doing exactly that. The suppliers are right. If the barrel in the door is already too long, it stands proud of the handle plate, and ordering the same size keeps it standing proud.

Lock Shop Warehouse recommends aiming for a protrusion of no more than 3mm, and the Suffolk Latch Company accepts a slight projection of 2mm to 3mm on each side of the handle plate. Tradelocks warns not to measure the existing cylinder because it may be the wrong size and cause overhang.

• Too long: the cylinder overhangs the handle plate, which is the exposure that makes a cylinder easier to attack.
• Too short: the cylinder sits recessed, the key can be awkward to use and the fixing screw may not line up.

Measure the door, check the old barrel against those figures, and treat any mismatch as a fault to put right rather than a size to copy. To choose a cylinder with a recognised mark, see the marks that tell you a cylinder is certified.

Three trade sources agree: Lock Shop Warehouse measures from the fixing screw centre to each end, the Suffolk Latch Company gives the 10mm cam and the two projections, and Tradelocks advises rounding to the nearest 5mm and not copying the fitted cylinder.

Sizing panel: three rules

1. Measure from the centre of the fixing screw hole, never from one end.
2. Round to the nearest 5mm.
3. Check the door, not just the old barrel. An overhanging cylinder is the weak point explained in our anti-snap locks guide.

A note on handles: guides disagree about whether the handle must come off. On most uPVC doors you can reach the screw with the handle still fitted. The handle only needs removing when the handle plate or escutcheon covers the screw or the face of the cylinder.

At this point you should be holding one screw, with the cylinder still in place. Never force or drill anything to get this far.

Two checks show whether the job is done properly:

• Projection: the cylinder should sit close to flush, standing proud by no more than about 3mm.
• Key action: the key should go in and come out smoothly from both sides.

Directional cylinders are marked so you can tell which face goes outside, as Tradelocks notes. A cylinder that will not fit without filing, forcing or packing is the wrong cylinder. If the old one turned out to be fine and something else was the problem, see whether the door needed a repair rather than a new barrel.

Yes, a cylinder can be removed without its key, and no, this guide will not explain how. In most cases the standard DIY removal method does not work without a key, because a working key is normally needed to line up the cam, and professional removal may be the answer.

The reason is simple: a method that removes a cylinder without its key works on any door, including one that is not yours. The outcome is what matters to you: the cylinder is destroyed and replaced, so the job ends with the new cylinder you were planning to buy anyway.

A snapped key is different and often better news. If part of the key is still in the cylinder, the cylinder itself may be fine once the broken piece is out; see getting a snapped key out of a lock or our.

Neither of these is the barrel. Treat the symptom as a pointer to what needs checking rather than a confirmed diagnosis.

• Door will not open, key turns normally: usually the mechanism or the alignment. The door may have dropped, or the hooks and bolts may no longer meet their keeps, and a new barrel will not touch that. Do not follow the replacement steps while the door is locked shut, and do not dismantle anything while the locking points are engaged. See a uPVC door that will not open.
• Key turns freely and nothing happens, handle does nothing either: this usually points to the gearbox in the middle of the mechanism, a different part in a different place, and a replacement must match the backset, PZ centres, spindle arrangement and fixings. See uPVC lock mechanism problems, and for a door that will not lock, why a uPVC door will not lock.

A new barrel bought for either fault is a part that was never the problem, and it usually cannot go back once it has been in a door. Never force a uPVC handle or key: that is how a working gearbox becomes a broken one.

Also stop if the door cannot stay open and safe, the correct part cannot be identified, or anything needs force; pressing on can turn a replaceable part into damage to the door, frame or whole locking system.