Chubb Manifoil Mk IV combination dial lock

This combination dial lock has been the UK government's security standard for many years.

 
 

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

• five turns clockwise to the first number
• four anti-clockwise to the second
• three clockwise to the third
• two anti-clockwise to the fourth
• then turn clockwise about half a turn until it opens

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.

 

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

• Open the lock. Always unlock the lock first, to check you have the correct original combination and are familiar with using it.
• Enter the first 3 digits of the combination. 
  Instead of the normal "Arrow" mark on the dial, use the straight line mark.
• Insert the setting key through the rear cover and turn it 1/4 of a turn anti-clockwise.
• Enter the new combination (3 numbers), again using the straight mark.
• Turn the setting key back, then remove it.
• Always test the new combination at least 3 times, 
  Before closing the lid !

Not all combinations are permitted.

• Do not use consecutive numbers
• Do not use all tens or all fives
• The fourth number is fixed and depends on the position of the bolt

  Bolt at the left (indicator at the top)

  Fourth number is 0 
  Don't use numbers between 95-10 for the third number

  Bolt at the bottom

  Fourth number is 25 
  Don't use numbers between 20-35 for the third number

  Bolt at the top

  Fourth number is 75 
  Don't use numbers between 70-85 for the third number

 

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.

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