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Letterbox lock replacement for a thin flap

Updated: 20.10.2026

The letterbox key is gone, or the cam no longer catches, and the post is either reachable or locked in. A letterbox lock replacement is a short cam lock in the existing hole, not a cage on the front-door letterplate. Measure the flap, pick a thin listing, and stop if the box is shut or the lock is a numbered series. The [cabinet lock comparison](/hub/burglar-protection/cabinet-lock) holds the five amazon.co.uk locks from 19 October 2026. We did not open a mailbox. A qualifying purchase can earn a commission at no extra cost to you.

The letterbox key is gone, or the cam no longer catches, and the post is either reachable or locked in. A letterbox lock replacement is a short cam lock in the existing hole, not a cage on the front-door letterplate.

Our top picks

VOCIBO VOCIBO 16 mm letterbox cam lock, 4 keys

Best overall

VOCIBO VOCIBO 16 mm letterbox cam lock, 4 keys
8.8/10about £5.99
View offer
shinfly shinfly 16 mm cam lock with 5 plates

Best value

shinfly shinfly 16 mm cam lock with 5 plates
8.4/10about £5.49
View offer
Lpraer Lpraer 30 mm combination cam lock, pack of 2

Alternative

Lpraer Lpraer 30 mm combination cam lock, pack of 2
5.8/10about £12.39
View offer
On this page
  1. 1.A mailbox lock is a small cam lock
  2. 2.Length and hole on a thin panel
  3. 3.If the box is already shut
  4. 4.Not the same as a front-door letterplate

A mailbox lock is a small cam lock

On a metal letterbox the cylinder passes through the flap and a small plate turns behind the frame. A stored VOCIBO review describes a 16 mm lock on a domestic postbox, called simpler than a recut key. The same search also returns letterplate guards and branded series that sell only coded keys. A generic cam lock stops a finger. It does not stop a tool on thin aluminium. Measure the hole, the flap, and the old cam. Body length is the cylinder. A square tail may not take a star-shaped cam: one VOCIBO review says it would not, which is why five plates are in the box. Four keys are copies for that lock, not the neighbour’s box.

Length and hole on a thin panel

The VOCIBO is £5.99, a 16 mm cylinder, a 19 mm drill and a panel up to 12.5 mm. It is the top pick here because letterbox replacements recur and the thickness is stated. The shinfly is £5.49, an 18 mm drill, a panel up to 12 mm, and plates at 3.2 cm and 4.7 cm. Drill 18 mm, not 17 mm from a photo. A 17 mm fibreboard hole would not take the body. The Litensh is 22 mm and states no panel thickness. The Lpraer title says 30 mm for 7/8 inch material, about 22 mm, which is a locker door. The measurement order is on the cabinet lock buying guide. Support thin metal so it does not dish, and test the plate with a card before a week of post.
VOCIBO VOCIBO 16 mm letterbox cam lock, 4 keys

Testsieger

VOCIBO VOCIBO 16 mm letterbox cam lock, 4 keys
8.8/10about £5.99
View offer

If the box is already shut

If the flap will not open, you need the cam worked, not a bigger drill. Drilling the plug can crease a thin door. Safety Letterbox describe drilling their own plugs and using a 12 mm socket and a 22 mm spanner. That is their method for boxes they know, not the first step on a communal bank. Advertised labour ran from about £20 to about £85 and is not a national fee. Ask before anyone drills. Bellsure replacements are series E, F, G and H at £22.50, and they will not send the key to the box address. A VOCIBO will not become an E-series lock because the hole looks similar. On a managed bank, check you are allowed to change it.

Not the same as a front-door letterplate

Fishing through a front-door slot is a different problem. A cam lock on a post box does not stop that, and a letterplate cage does not lock a mailbox flap. For a gate or hall box, use a cam lock only when the flap is open and under about 12 mm. The VOCIBO’s 12.5 mm and the shinfly’s 12 mm are the two figures here that match. Keep a spare key out of the box. It is casual privacy, not a burglar-proof letterplate.

Common questions

I lost the letterbox key. Can I buy a cam lock and fit it?
Yes, if the flap is open or you can remove the old lock and measure the hole and the thickness. The VOCIBO and the shinfly are the thin-panel listings here, up to 12.5 mm and 12 mm. If the flap is shut and the post is inside, get it opened before you buy a cylinder on a guess.
What length suits a thin flap?
A short body, in this set the 16 mm class, with a stated panel limit around 12 mm. The VOCIBO states up to 12.5 mm. The shinfly states up to 12 mm. A 22 mm or 30 mm body is for a thicker door unless you have measured that thickness on this flap.
The door is shut. Do I drill the lock?
Not as the first step, and not while the mail still matters. Drilling destroys the cylinder and can damage a thin flap. One specialist guide does drill their own boxes. For a box you do not know, a locksmith opens it, then you measure. Agree the price first. Advertised figures ran from about £20 to about £85 and do not average into one national fee.
Will any 16 mm lock fit a Bellsure or communal bank?
No. Bellsure replacements are series E, F, G or H at £22.50, with two coded keys, and a photo if you are unsure. A generic 16 mm cam lock is for a plain hole you have measured. On a managed bank, check you are allowed to change the lock.
Are several plates why these listings suit letterboxes?
Yes. The VOCIBO and the shinfly each include five plates, so you can match the reach to the frame instead of reusing a cam that may not fit the new tail. One stored review could not put a star-shaped cam on a square tail. Try the plate with the flap open before you lock the first week of post inside.
Is this the same as a letterplate guard on my front door?
No. A letterplate guard or cage is for fishing through the slot in an entrance door. A mailbox cam lock only holds a separate flap shut. Buying one does not solve the other.

Related pages

How to fit a cabinet lock

Best cabinet lock

Cabinet lock

Cabinet lock buying guide: measure before you order

Keyed alike vs keyed different for cupboard door locks

Sources

Public UK sources used for measurement, keying and the legal limit. Listing prices, stars and review counts are from amazon.co.uk on 19 October 2026.

  1. Lockshop Direct — how to measure a cam lock (retrieved 19 October 2026)
  2. Lock & Key Centre — choosing cam locks, KD, KA and rotation (retrieved 19 October 2026)
  3. G Johns — keyed-alike cam locks for cabinet doors (retrieved 19 October 2026)
  4. BSI Knowledge — BS 3621:2017+A1:2024 scope, building doors (retrieved 19 October 2026)
  5. legislation.gov.uk — Misuse of Drugs (Safe Custody) Regulations 1973, Schedule 2 (retrieved 19 October 2026)
  6. HSE — flammable-liquid storage in workrooms (retrieved 19 October 2026)
  7. Intertek Inform — BS EN 16948:2017 child-resistant cupboard devices (retrieved 19 October 2026)
  8. Locktrader — cabinet and cam locks, nut-fix and snap-fix (retrieved 19 October 2026)