Chubb Manifoil Mk IV combination dial lock

last update: December 2004

Chubb Manifoil Mk IV combination dial lock

Front view of dial

This combination dial lock has been the UK government's security standard for many years. Mine was acquired on a government surplus steel case. Although unused, it is stamped '1967'.

steel document case

The lock has the normal operation for a combination lock. The combination has four numbers and is always dialed in the same way;

Although there are four numbers in the combination, only three are significant and settable. The fourth is fixed and will always be 0 for a right-swinging lock, or 25 for a top-swinging door like this.

rear view - upside down

This is the rear of the lock, when open, but with the bolt returned to the "locked" position. Once unlocked, the bolt can be moved back and forth repeatedly, by moving the dial about 1/4 of a turn.

Note the "broad arrow" marking, typical for UK government issue. The rear case is held on by two screws. Normally these would be lock-wired and sealed with a lead crimp when in service.

Later versions have a "hook" inside the case that prevents it being removed unless the lock is unlocked first, even if the screws are undone.


Setting the combination

Setting Key

Setting the combination uses a key, inserted through the small covered flap on the rear cover.

Not all combinations are permitted.


Inside the mechanism

Mechanism - Cover open Mechanism - Cover open

Most of the lock's operation centers on the cylindrical disk pack in the lower half of the lock. This is driven by the dial, but revolves backwards relative to it.

The disks carrying the combination are invisible beneath the outer disk. The first number is on the most deeply hidden disk. If the safe is open, but the lock combination is unknown, it is possible to open the lock by inspection of these disks with a small torch.

When the combination is entered, the notches in the disks line up and allow the comb to fall into the notch. The final half-turn rotates the outer disk, so that the visible notch in it catches the large lever and pulls it down, opening the bolt.

Mechanism - locked

Mechanism - unlocked

Several aspects of the mechanism are notably sophisticated. There are even a pair of lead shields to prevent X-ray examination.

Photo sequence of a disassembled lock