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Mervyn Passmore 2005
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Where the original clock chain is present and fits the clock, a suitable replacement can be found by measurement and comparison with illustrations of new chains. In cases where the chain is missing, or the fitted chain does not match the sprockets, the following formula should establish the nearest chain to use:

Count the total number of sprockets on the chain wheel.
Our example has 8:

Measure the effective diameter of the pulley excluding the sprockets, in mm.
If the sprockets are wider at the base, as in our example, the effective diameter is the lowest point on the sprockets reached by horizontal links. This should be apparent by wear marks.
Our example is 30 mm.

Multiply the diameter by 3.14.
Divide the answer by the number of sprockets.
Divide this answer by 2.
Multiply that answer by 1.1.

Our example:
30 x 3.14 = 94.2
94.2 / 8 = 11.78
11.78 / 2 = 5.89
5.89 x 1.1 = 6.48mm internal link length.

Refer to the list of chains available that has an internal link length close to the answer, and whose external link width will fit well between the sides of the chain wheel. If you select a chain link shorter or much larger than the answer, it will probably 'ride up' over the sprockets.
Clock chain is one of the most difficult items to select and purchase by mail order. Frequently the sprockets are worn, or the wrong chain has been fitted. We get samples sent to us like bathroom chain which of course we cannot match. If you have got a piece of chain that fits, measure the internal link length. Refer to the list, and verify your choice by placing it over the drawing.

If you have no chain, or the one you have does not fit, make sure that the clock is not designed for rope. There must be somewhere for the alternate links (the ones not over the sprockets) to go. Rope driven clocks don't have this, and you cannot just fit chain to a rope clock. Rope driven clocks often have sprockets that are not carefully set. Chain sprockets have to be equally positioned to match the chain pitch - rope sprockets can be anywhere around the circumference.