A double cylinder deadbolt that is hard to lock is one of the most common locksmith call-outs in New Zealand — and in many cases, you can sort it yourself in under an hour. The most likely culprits are a misaligned strike plate, a dry or dirty bolt mechanism, a sagging door, or a worn key. Work through the checks below in order and you will usually find the problem quickly.
Understand How a Double Cylinder Deadbolt Works
A double cylinder deadbolt requires a key to lock and unlock from both sides of the door — unlike a single cylinder, which has a thumb-turn on the inside. Because both sides rely on the key engaging the cylinder and turning the cam, any stiffness in the cylinder, bolt, or strike plate alignment will make the lock feel heavy or jammed. Understanding this helps you pinpoint where the resistance is coming from.
Step 1 — Try a Different Key
Before touching the lock, try a spare key. A worn or bent key is a surprisingly common cause of a stiff deadbolt. If the spare turns smoothly, your original key needs replacing. Take the working key to a locksmith or key-cutting service to have a fresh copy cut.
Step 2 — Lubricate the Lock Cylinder
Dry cylinders are the number-one cause of stiff deadbolts in NZ homes, especially after a dry summer or in coastal areas where salt air accelerates wear.
- Use a graphite-based lubricant (available in a powder or spray form) or a PTFE (Teflon) spray — both are suitable for lock cylinders.
- Avoid WD-40 or general-purpose oil sprays. They attract dust and grime over time, making the problem worse.
- Insert the nozzle into the keyhole and apply a small amount, then insert the key and work it in and out several times to distribute the lubricant.
- Do the same on the bolt mechanism — spray a little lubricant where the bolt slides in and out of the lock body.
Graphite spray and PTFE lubricants are available at Bunnings and Mitre 10.
Step 3 — Check the Strike Plate Alignment
If lubrication does not fix the problem, the strike plate (the metal plate on the door frame that the bolt slides into) is likely misaligned. This is especially common in older NZ homes where timber frames have moved with seasonal moisture changes.
How to check alignment
- Apply a small amount of lipstick or crayon to the end of the bolt.
- Close the door and turn the key so the bolt extends into the strike plate hole.
- Open the door and look at where the colour has transferred onto the strike plate — this shows exactly where the bolt is hitting.
Fixing a misaligned strike plate
- Minor misalignment (up to 3 mm): Use a metal file to enlarge the strike plate opening in the direction needed. This is a simple DIY fix.
- Moderate misalignment (3–6 mm): Unscrew the strike plate, chisel the mortise slightly in the required direction, and refit the plate with longer screws (at least 65 mm) to bite into the stud behind the frame for added security.
- Significant misalignment: The door itself may have dropped or the frame may have shifted — see Step 4.
Step 4 — Check for a Sagging or Swollen Door
NZ's variable climate — particularly in high-humidity regions like Auckland, Northland, and the West Coast — causes timber doors to swell in winter and shrink in summer. A door that has dropped on its hinges will also throw the bolt out of alignment with the strike plate.
- Hinge screws loose? Tighten them. If the screw holes are stripped, remove the screws, plug the holes with a wooden toothpick and wood glue, let it dry, then refit the screws.
- Door swollen? You may need to plane or sand the door edge slightly. If the door is painted, repaint the bare timber afterwards to prevent moisture re-entry.
- Structural movement? If the frame itself has racked or the house has settled significantly, consult a builder or Licensed Building Practitioner.
Step 5 — Inspect the Lock Mechanism Itself
If the bolt still feels stiff after lubrication and alignment checks, the lock mechanism may be worn or damaged internally.
- Remove the lock from the door (usually two screws on the interior face plate).
- Operate the bolt by hand and by key with the lock out of the door. If it is still stiff out of the door, the mechanism is worn and the lock should be replaced.
- Double cylinder deadbolts from reputable brands such as Lockwood, Gainsborough, or Whitco are widely available at Mitre 10, Bunnings, and PlaceMakers.
Step 6 — Re-key or Replace the Cylinder
If the cylinder itself is worn (common after many years of use), a locksmith can re-key or replace just the cylinder rather than the whole lock, which is a cost-effective option.
Safety Note
Double cylinder deadbolts provide excellent security but can be a fire safety risk — if a key is not readily accessible from inside, occupants may be unable to exit quickly in an emergency. Always keep a key hanging near (but not visible from) the door when you are home. Check with your local fire authority or building consent authority if you are unsure about egress requirements.
When to Call a Professional
Call a qualified locksmith if:
- The lock is still stiff after lubrication, alignment correction, and hinge tightening.
- The cylinder is damaged, seized, or the key will not insert at all.
- You need the lock re-keyed to match other locks in your home.
- The door frame has shifted significantly and needs structural assessment.
- You are a landlord — under the Healthy Homes Standards, rental properties must have secure, functioning locks on all exterior doors.
A qualified locksmith can diagnose and fix most deadbolt issues in a single visit. You can find trusted local locksmiths on tradedirectory.co.nz.
