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Why are not all clock keys stamped #6 are the same size in mm's across the different brands?
The Fundamental Problem: Multiple Numbering Systems
The number sizing system seems to have evolved many years ago with the number on the key corresponding to a size on a chart. This system comes unstuck when you find out there's at least three different scales, all slightly different, starting from different points, meaning a No 8 key made by one manufacturer will not be the same as a No 8 by another.
The Three Main Systems
1. American/English System
- Uses numbers 000 through 16
- Increments by approximately 0.2mm between sizes
- Most common in American-made clocks (Seth Thomas, Ansonia, New Haven, etc.)
- Based on historical American clock manufacturing standards
2. European/Swiss/Continental System
- Also uses numbers (sometimes different ranges)
- Increments by 0.25mm (one quarter millimeter) between sizes
- Used for European clocks (German, Swiss, French)
- The keys in the metric system rise in 0.25mm (one quarter millimetre) increments, 3.25mm, 3.50mm, 3.75mm, 4.00mm and so on.
3. English (Traditional) System
- A third, older British system
- The European numbering system is standard these days, and that is how our keys are marked, but you can see all three scales (European, English, millimetres) on the charts below.
Why the Numbers Don't Match Across Systems
There are two different sizes, Swiss and American, and they are not the same.
The key issue is that each system started from a different baseline and uses different increments. So:
- An American #6 might measure 3.65mm
- A Swiss/European #6 might measure 3.50mm or 3.75mm
- An English #6 might be yet another measurement
This means a key stamped "#6" could be three different actual sizes depending on which manufacturer made it and which sizing system they followed.
Historical Reasons for This Chaos
1. No International Standards
The key size chart has all of the sizes listed however sometimes certain key sizes are known. Generally speaking, almost all post 1960 mechanical clocks, made in Germany take a size #8 key or crank, unless the movement is round.
When clock manufacturing developed in different countries (America, England, Germany, Switzerland, France) in the 18th and 19th centuries, each region developed its own sizing conventions. There was no international horological standards organization to unify these systems.
2. Arbitrary Number Assignment
Clock keys range from 000 to 16, but the number doesn't directly relate to the width in millimeters, which can be confusing.
Clock keys have there own number system from 000 to 16 as seen in the Clock key size chart. However the number does not indicate what size is what number and can be confusing.
The numbers were assigned somewhat arbitrarily to sizes that were common in each manufacturing region, rather than being based on actual measurements.
3. Manufacturing Tolerances and Variations
Harry also points out that sizes of keys from different manufacturers are not consistently accurate.
Personally, I have ordered keys that were supposed to be the same size, but they weren't.
Even within the same system, different manufacturers had slight variations in their actual production, meaning two keys both marked "#6 American" might not be exactly the same size.
Additional Complicating Factors
Tapered Winding Arbors
The other thing, which adds to the difficulty of sizing, is that older clocks tend to have tapered winding squares, matched by tapered mortises in their (antique) winding keys.
It is worth remembering that most winding arbors taper out towards the clock, and the end section is most likely the smallest. This is advantageous because it makes the selection of the size less critical, up to a point anyway, as the key will slide onto the shaft until it reaches the ideal point.
Older clocks (especially pre-1900) often have tapered winding arbors that get wider toward the clock face. This means the "correct" key size depends on where on the taper it's meant to grip.
Different Measurement Standards (Imperial vs. Metric)
While modern measurements are in millimeters, older American and English sizing systems may have been based on fractional inches, then later converted to metric, creating odd decimal equivalents.
The Modern Solution: Direct Metric Measurement
The Metric sizing system is a much more straight forward way of selecting the size of key you require. The size stamped on the key refers to the size of the internal square in the key (across the flats) meaning if you have a clock with a 4.00mm winding arbor, you just buy a key to fit.
Still confused? Forget about the different systems of numbering keys, just measure it in mmm (millimetres) - all millimetres are the same.
Many modern suppliers now mark keys with actual millimeter measurements rather than arbitrary numbers, which eliminates the confusion.
Practical Implications for Clock Owners
Always Measure, Don't Assume
A 7 key is shown as 400mm but I have bought 7 keys that are about 370 mm. Are there two systems defining key dimensions and related sizes?
This comment (which likely means 4.00mm vs 3.70mm) illustrates the problem perfectly - even a "#7" can vary significantly.
Know Your Clock's Origin
Generally speaking, almost all post 1960 mechanical clocks, made in Germany take a size #8 key or crank, unless the movement is round. The American time strike antique units mostly take the size 7 key.
Understanding whether your clock is American, German, Swiss, or English helps narrow down which sizing system applies.
Use Conversion Charts Carefully
When using conversion charts, make sure the chart specifies:
- Which numbering system it's using (American, Swiss, English)
- The actual millimeter measurements
- The clock's country of origin
Consider Buying Multiple Sizes
Considering that keys are only a couple of bucks each, it's pretty cheap to get a good assortment of different sizes.
Since keys are inexpensive and the sizing is confusing, many collectors keep an assortment of keys in adjacent sizes.
Summary
A clock key stamped "#6" is not the same size across different brands because:
- Three different numbering systems exist (American, European/Swiss, and traditional English)
- Each system uses different increments (0.2mm vs 0.25mm) and different starting points
- No international standardization occurred when these systems developed in the 18th-19th centuries
- Manufacturing tolerances vary between makers even within the same system
- Historical measurement systems (imperial vs. metric) created conversion inconsistencies
- Tapered arbors on older clocks add another variable
The only reliable method is to measure the actual winding arbor in millimeters and order a key based on that measurement, or use a metric-stamped key that shows the actual millimeter size rather than an arbitrary number.
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