Pendulums are available for almost all types of clocks and the correct length is often stamped on the back-plate of mass produced clocks. American clocks are in inches, German in mm, and French in lignes.

How to calcuate the length of a French clock pendulum from the numbers on the backplate

Many French clocks have the pendulum length stamped on the backplate, in the French pre-metric 'Pouce & Lignes' system. You may find a number such as 4.2

If you multiply the first part by 27.07 (the millimetres in a Pouce) and the second part by 2.256 (the millimetres in a Ligne) you will find the original pendulum length.

To take our example of 4.2:

4 * 27.07 = 108.28

2 * 2.256 = 4.512

108.28 + 4.512 = 112.792mm

Of course this is far too accurate a conversion, but we know the pendulum in our example should be between 110 - 115mm long. When cutting a new pendulum rod, it is always wise to cut it on the long side, with the Brocot adjuster up. Drill out the hook to be a tight friction fit and test the formula before finally cutting to length and riveting the hook in place.
The length calculated is from the top of the flexible part of the suspension spring to the centre of mass of the bob. In practical terms this is the centre of the bob, but in theory it will be slighlty above the centre due to the rod. 

French clocks usually have the movement serial number stamped on the pendulum. If there is any discrepancy in this and the number on the backplate, and the clock cannot be regulated, it may not be the correct pendulum.

French pendulums are available in two types:

Silk suspension - A small brass disc on the end of a fine rod hangs from the silk thread at the top. A brass block is on the rod, level with the crutch forks.

Metal suspensions - Three sizes are available

  • Size '0' is normally used on small timepieces
  • Size '1' is used on normal striking movements
  • Size '2' is used on larger than normal striking movements

There are two ways to fit the hook to one of these pendulums

Cut the rod to the correct length and then either:

  1. Thread the rod and tap the hole in the hook. For a professional fit, tap and thread using a 2.5mm x 0.45mm tap & die.
  2. Enlarge the hole in the hook and reduce the diameter of the rod. Glue or solder them together.

Why doesn't a new hook fit a new rod?

A new French hook is designed to fit any rod, not just new ones, so the hook and its hole has to be small.

A new rod is designed to fit most common bobs, so has to be larger than some original rods.

To make them fit each other, enlarge the hole in the hook as much as possible without weakening it. Reduce the diameter of the rod where it goes into the hook with a flat file. Some repairers thread both items to make a quality fit. Others make the items a friction fit and either silver solder them apply a little adhesive.

A supplier cannot reduce the diameter of the rod for you because you will have to cut it to length first. 

Repairing Your Own Clocks by Mervyn Passmore