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Making a Clock Mainspring

by Mervyn Passmore

As the range of suitable, ready-made clock mainsprings available to repairers diminishes, the need to make or modify a spring is becoming more common. It is vital that a spring is made correctly, using appropriate material. Wrongly softened or badly shaped holes can cause a premature fracture, which can be very damaging to a clock train, and a spring that is too weak or strong, can be equally problematic.

Tools required to make a clock mainspring are:

Selecting suitable material

The original clockmaker will have designed the size of the barrel and the gear train based on many criteria including the hardness (strength) of the mainspring material. To maintain authenticity and to avoid damage to the train, only traditional carbon steel spring material should be used as a replacement.

The motor industry is currently under enormous pressure to reduce carbon emissions. If they can find steels that are more powerful than traditional materials, a spring’s thickness can be reduced. This in turn reduces the overall weight of the vehicle, which can put it into a different CO2 group. This has meant that steel rolling mills now concentrate on making very high powered springs for items such as car seat belt re-coilers and other parts that use traditional coiled steels.

The use of high power spring steel in clocks is causing damage to clocks and should be avoided. Whilst it is possible to replace a traditional spring with a thinner gauge of high power material, this is undesirable from a professional point of view. It destroys authenticity, the required percentage reduction in thickness is extremely difficult to calculate and the substitution will cause problems for the next repairer.

Research has shown that the best specification for a suitable material for clocks made before 1970 is in the now obsolete British Standards BS5770: Pt. 3: 1981.  Although this uses a somewhat outdated hardness test system, steel that fails this test can generally be considered to be too weak or too strong for old clocks. Always ensure that any springs and material you buy complies with BS5770.

Spring weakness can also be a problem, although unlikely to damage the train. This is much easier to test if you do not have access to a hardness tester, but cannot be done until a spring has been made and wound. It is a destructive test.

A few exceptions apply. For example Konrad Mauch made a large number of 400 day clocks in their Miniature and Midget ranges with a mainspring that had insufficient power. The movement was re-issued with a 15% wider barrel, which would have increased the force by 15%. This is an excellent example of where a higher powered spring could be useful to a repairer.

M&P Spring Material

Calculating the length

Material dealers supply springs by specifying the barrel diameter. This is because the length of an existing spring cannot always be trusted. An antique clock will have survived two world wars and several recessions. If the spring has broken during a period when replacements were difficult to obtain, and the break is near one end, it is likely that a repairer will have discarded the broken part and made a new end. This practice is acceptable if the fracture is very close to or involves the hole. However, larger pieces are sometimes discarded and clocks that were designed to run for 8 days will have a reduced period of operation.

Similarly a spring may have been replaced with one that is too short if the correct replacement item was unavailable at the time.

You should either rely on your material dealer’s expertise or do the calculations yourself. Our mainspring calculation tool may be of assitance here. Visit the website at and select 'technical help' along the left hand side, and scroll down to Online Calculators and select 'Calculate the Correct Mainspring Length'.

Softening the ends

The two ends of a spring need to be softened in order to make the holes and also to ensure that the material engages correctly in the hooks. Softening is a simple process but requires very careful attention. The process involves heating the final few centimetres to red heat and cooling it slowly.

Ideally, you should heat the end in an enclosed, heat reflective area such as in a group of fire bricks. It is important that the steel can cool off slowly. If it cools suddenly due to a blast of cool air or being placed on a cool surface it can harden and become brittle.

Using a gas torch, heat the end until it is dull red but not yellow. Reduce the heat and wait until the colour has completely disappeared. Allow to cool slowly. Remove any discoloration with a flap wheel or an abrasive block such as a Garryflex block.

The shape of the hole

A repairer will have seen almost every permutation of hole shape during his or her working life. Round, square and pear shaped, large and small.

Research carried out with the help of the British Horological Institute identified the following criteria:

A hole in a spring must have no sharp corners, because these will encourage a tear to start.

It must not have a curve on the leading edge that forces the spring to self-centre in the barrel. The barrel hook is not always in the centre.

The hole needs to be as small as possible.

The obvious conclusion is the traditional pear-shaped hole that has a flat surface on the leading edge and has the narrow end facing the main part of the spring. This shape complies with all the required criteria, and is the professional’s choice. Any other shape should be considered as undesirable

Hole Making and Scribing

To make a hole select a suitable hole template - the old spring is ideal for this purpose. Place it on the end of the material and scribe around the hole and the outer curve. Using snips, carefully cut around the arc that defines the end of the spring. File away any burrs.

M&P Spring Template KitM&P Spring Preparation

Using a punch tool if you have one, punch out as much as possible of the material inside the scribed line.

We now sell an easy-to-use multiple sized, circular hole punch which can be used to begin the end making process:

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M&P Spring Tool

If you do not have a punch tool, drill a hole. To get started you may need a centre drill; without one the drill bit tip may wander around the surface.

Once you have a starting hole, you can file away the interior of the scribed hole so that it exactly matches the template. Finally, it is important to add a short, soft curve to the outer end of the spring to match the curvature of the intended barrel. Also, add a slightly sharper curve to the inner end to match the curvature of the intended arbor. The curve is essential, particularly if the spring is to be inserted by hand.


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