Always use the correct key for a clock. Standard keys are available in
brass or steel. Generally, keys made from brass are preferable to those made
from steel because they are softer. It is a better plan to wear the key down
over time than wear the clock arbor. Brass keys are normally fatter than steel
ones, because the material is weaker, so you may need to use a steel one,
espacially on French clocks where the hole in the dial is minimal.
There
are two numbering systems to be found, in addition to the basic dimensions
across the flats. The English system increments differently to the Continental
one and as keys are often only marked with a number you cannot normally tell
which system it refers to. Referring to a key by number is therefore almost
meaningless. The old numbering systems have now been replaced by metric
measurement of the arbor across the flat surfaces. When a clock is dismantled,
examine the flats of the winding arbors and square them off with a file if
necessary. Measure across the flats if a new key is needed, or try a bunch of
new keys to find the best fit. A loose key will damage and round off the corners
of the arbor.
A winding Key Sizing Guage is a useful tool for measuring old keys.
Repairing Your Own Clocks by Mervyn Passmore