Enter your observations
Use measurements you can verify. Unknown or unreadable details can be left unspecified where the tool allows it.
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How to use this tool
- Measure one flat side of the square arbor with a suitable measuring tool. Do not measure diagonally across the square.
- Enter the value in millimetres or, alternatively, inches. If using inches, leave the millimetre field blank or disregard it.
- Choose the chart convention that matches the supplier or historical reference you are using.
- Use the nearest size only as a candidate and verify the key socket fit gently.
What the result means
Clock winding keys are commonly described using numbered sizes, but conventions differ. The reference tables used here list European sizes from 000 (1.75 mm) through 18 (6.75 mm), and an older British sequence from 000 (2.0 mm) through 18 (6.0 mm). Values are nominal chart references, not universal manufacturing tolerances.
Correct measurement is essential: measure across one flat side of the square arbor, not corner-to-corner. The arithmetic converts inches to millimetres (25.4 mm per inch) and returns the nearest chart sizes so a collector can compare rather than guess.
Limits and good practice
Clock-key charts may vary by maker, period, and source. A dimensionally close key may still have the wrong socket profile or quality. Do not force a key, polish the arbor or reshape historic components based on this output; consult the clock maker or a horologist for a valuable movement.
References and further research
Frequently asked questions
Should I measure across the square diagonally?
No. Measure the flat side of the square arbor. A diagonal measurement will overstate the size.
Why are British and European sizes different?
They are different nominal size sequences from different sizing conventions. Always match the chart to the source or supplier.
What if the arbor measures between two sizes?
Recheck the measurement and inspect for wear. Do not assume the larger size is correct; a clock key should fit without force or excessive play.
Can I enter inches?
Yes. Enter inches in the alternative field and the tool converts using 25.4 mm per inch. Avoid entering conflicting values in both fields.
Do all makers use this chart?
No. Nominal charts and tolerances may vary. Confirm with a specialist supplier when the movement or key is unusual.
Will a correctly sized key operate the clock?
Not necessarily. The movement may be damaged or have an unusual arbor. Size is only one aspect of compatibility.