Clock key sizes – do you have the right one?

Having the correct key for your vintage or antique clock is important. When you buy a clock it generally comes with a key. Since they are so often lost at some point or another it is likely a replacement and it usually fits. Occasionally you will acquire a clock without a key and it means searching for the correct one. If you have other mechanical clocks in your house you can try those keys and one will usually work. Failing that you can use a micrometer to measure the winding arbors (or shaft) for a precise fit and order one online. However, this often means taking the movement out of its case.

Keys come in all shapes and sizes. In the photo above you can see keys from German and American movements. The two pronged keys (4 and 5 prongs) are available at any clock supply house such as Perrin or Timesavers and will fit just about every clock winding arbor. Many other sizes cab also be ordered. In the photo below you can see two types of double-ended keys.

The one on the left is for winding a typical American clock and the small end is for adjusting the speed of your clock. Many clocks have a smaller arbor used for adjusting the speed of your clock and will have “S” and “F” (Slow/Fast) inscribed on the dial generally near the 12 o’clock position. The one on the right is for a time-only carriage clock. The small end is used to advance the minute hand by making an adjustable from the back of the clock.

Read the below chart to determine the correct sized key to wind your clock. In general most larger German movements take a number 8 key but some take a number 4.

If you have a 31 day clock, or a Korean/Chinese clock, or a commonly produced early American time & strike movement, then it usually takes a number 6 or 7. However, you want to be sure by checking the following chart.

I keep the key either beside the clock or in the case of a wall clock, inside the case. Some mantel clocks have key caddies but I find them a hassle because you have to move the clock and open the back door to access the key. On the other hand, if you have little kids it might be a good idea.

All sizes in mm starting from small to large


 Key size..........American................. Swiss
 5/0.................. 1.6...................1.25
 4/0.................. 1.8 ..................1.5
 3/0.................. 2.0.................. 1.75
 2/0.................. 2.2.................. 2.0
 0.................... 2.4 ..................2.25
 1.................... 2.6.................. 2.5
 2.................... 2.8.................. 2.75
 3.................... 3.0.................. 3.0
 4.................... 3.2...................3.25
 5.................... 3.4.................. 3.5
 6.................... 3.6.................. 3.75
 7.................... 3.8.................. 4.0
 8.................... 4.0.................. 4.25
 9.................... 4.2.................. 4.5
 10................... 4.4 ..................4.75
 11................... 4.6.................. 5.0
 12................... 4.8.................. 5.25
 13................... 5.0.................. 5.5
 14................... 5.2.................. 5.75
 15................... 5.4.................. 6.0
 16................... 5.6.................. 6.25
 17................... 5.8.................. 6.5
 18................... 6.0.................. 6.75
 19................... 6.2.................. 7.0
 20................... 6.4.................. 7.25
 21................... 6.6.................. 7.5
 22................... 6.8.................. 7.75
 23................... 7.0.................. 8.0

For weight driven clocks you will either have chains which involves pulling the weight(s) to the top of the clock once each week or a winding crank (photo above) to bring the weights up. Simply insert the crank into the arbor and wind s.l.o.w.l.y clockwise.

Having the right key is important and they are not hard to find if you either lose your key or your clock comes without one.