Clock key sizes – do you have the right one?

This article will explain what the clock key is for and why the correct size for your clock is important.

Mechanical clocks require periodic winding. Some must be wound once a day but most are wound once per week. A key that fits well should not be tight or difficult to insert but it should have a snug fit. No matter how many winding arbours or winding points you have on a clock one key will fit them all. 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 well.

If you want to know how to wind a mechanical clock check out this article

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. You can also use a micrometer to determine the thickness of 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 are 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 arbour. Many other sizes can also be ordered. In the photo below you can see two types of double-ended keys.

The one on the left is used for winding a typical American clock. The large end fits into the winding arbour while the small end is for adjusting the speed of your clock. Many clocks have a smaller arbour 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 key on the right is for a time-only carriage clock. The small end is used to advance the minute hand and the large end fits onto the mainspring arbour.

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, it usually takes a number 6 or 7. However, check the following chart.

30-hour weight driven Ogee clocks require a winding crank and should not be wound with a key.

keys should be kept either beside the clock, in the case of a wall clock, or on a caddy inside the case. Some mantel clocks have key caddies but they are a hassle because the clock must be moved to open the back door to access the key. If you have small children keeping the keys out of their reach is a good practice.

I have included a chart to determine the correct sized key needed to wind your clock.

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

Weight driven clocks that have chains involve pulling the weight(s) to the top of the clock once each week or for those with cables, a winding crank (photo above) to bring the weights up. Simply insert the crank into the arbor and s.l.o.w.l.y wind the weights up.

Having the correct key is important. They are not difficult to locate if they are lost or your clock comes without one.

If you want to know how to wind a mechanical clock check out this article.