Clock key sizes – do you have the right one?

This article will help explain what the clock key is for and why the correct size for your clock is important. It also contains a chart to determine the correct size key for your clock.

Mechanical clocks require periodic winding. Some must be wound once a day (30-hour clocks) but most are wound once per week (8-day clocks). A few run 14 days and anniversary clocks will run up to 400 days.

Most mechanical clocks require that a key be inserted into a winding hole located on the dial face. A key that fits well should not be tight or difficult to insert but it should have a snug fit.

With the exception of the smaller regulating arbour found on some clock faces all winding arbours or winding points on the clock face are the same size and one key will fit them all. Clock keys often go missing in the life of a clock and the key you get when you buy a clock is likely a replacement and it usually fits well.

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

A clock without a key 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.

A micrometre is useful to determine the thickness of the winding arbours (or shaft). However, this often means taking the movement out of its case. Measure for a precise fit and order one online.

Using a micrometer to check pivot
Using a micrometre to check pivot diameter; this tool can be used to measure the thickness of a winding arbour
4 and 5 prong keys and singles
4 and 5 prong keys and singles

Keys come in all shapes and sizes. In the photo below 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 many clock winding arbours. Many other sizes can also be ordered. eBay, online for-sale sites and antique stores are other sources for clock keys.

In the photo below you can see two types of double-ended keys.

Double-ended keys
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.

Speed adjustment arbor
Speed adjustment arbour requires the small end of a two-ended key

In general, most larger German movements take a # 8 key but some take a #7.

31-day clocks, or a Korean/Chinese-made clock, usually takes a number 6 or 7 key. Many American-made mantel clocks use a #5 or #6 key.

Many tall-case (grandfather clocks) require a #10 or #11 winding crank and should not be wound with a key. For those with weight cables, a winding crank is used to bring the weights up. Insert the crank into the arbour and s.l.o.w.l.y wind the weights up to the top of the clock so that they are still fully visible. 30-hour weight-driven Ogee clocks require a #4, #5 or #6 crank key.

Some clock keys will also have a number stamped on them indicating their size.

Keys should be kept either beside the clock, within the front access door of a wall clock, or on a caddy inside the case. Mantel clocks that have key caddies are a hassle because the clock must be moved to open the back door to access the key. However, if you have small children, keep the key on its caddy so as to be out of their reach.

The chart below gives the correct sized key needed to wind your clock.

Key caddy behind access door
Key caddy behind the access door
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
Winding crank
Winding crank in a miniature Vienna Regulator

There is no key required for weight-driven clocks that have weight chains. Winding involves pulling the weight(s) to the top of the clock once each week.

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

check out this article on How to Wind A Mechanical Clock


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