Hole end spring

Always look carefully at an old mainspring before ordering a new one. The range of replacement springs has increased enormously over the past 20 years. It could be the original has been replaced before, using the nearest that could be obtained at the time. During two world wars springs were hard to obtain. A broken end was often discarded and a new eye made in the remainder.

The length of a spring can be calculated as follows:

The spring should occupy 1/3 to 1/2 of the free space inside a barrel. This calculation assumes 1/2.

80% of the length calculated will be sufficient for the clock to run.

Length = (((3.142 x (D/2) x (D/2)-(3.142 x (d/2)* (d/2)))/2)/t

Where D=Internal barrel diameter. d=arbor diameter and t=thickness.

If the nearest spring you can obtain is too long, you can shorten it. Heat the point to shorten it at to red heat in a flame. This will soften it. When cool, simply cut of the surplus and fashion the end and the eye to match the original. Heating the right amount of material to the correct temperature does require some expertise and should only be done when absolutely necessary. If not softened enough, it may fracture. If too soft it will tear. Always finish off the curves of the eye well, as a hairline fracture here will eventually tear.

In the same manner, you can make a new end on an old spring that has broken off at the outer end. Experience shows that once a spring reaches the stage when it breaks in use, it will do the same again. As a mainspring breakage can cause so much damage, it is not good practice to re-make the ends.

Loop end springs are measured by:

• Height or width in mm.
• Strength or thickness to the nearest 1/100mm. 
• Length in mm.

Loop ended spring

Although most American clocks were originally fitted with riveted end springs, it is much easier to replace them with open loops. Riveted ends can be removed simply by heating to red heat and cutting off with snips.
Assemble the clock with no springs. This makes it easier to get all the striking train in the correct sequence. Simply fit the inner end to the arbor, and wind most of the material onto the clock. Slip the open loop over the pillar, ensuring that the tail fits between the coils.