Anniversary clocks, also known as 400-day clocks, are marvels of horology. Their long-running torsion pendulum mechanism relies on a slender strip of spring steel known as the suspension spring. This tiny component regulates the timekeeping of the clock, and choosing the correct replacement size is critical for accuracy. In this guide, we’ll explain how suspension springs work, how to identify the correct size, and how to fit them properly to restore a 400-day clock to perfect operation.
What Is a Suspension Spring?
A suspension spring is a flat, ribbon-like spring that suspends the torsion pendulum beneath the movement. Its thickness, length, and assembly all affect the clock’s timing. Too thick, and the clock will gain time. Too thin, and it will lose. Even a variation of 0.0001" in thickness can significantly alter performance.
Identifying the Correct Size
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Back Plate Reference: Most 400-day clocks can be identified by the back plate of the movement. Matching this to the Horolovar repair guide will tell you the exact spring size required.
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Common Thicknesses: Ranges from 0.0020" to 0.0032". The most widely used are around 0.0023"–0.0025".
- Length and Assembly: Suspension units include the top block, fork, and bottom block — all must match the original.
If you're not sure which size you need, please contact us with a clear image of the clock's back plate and we will identify it for you.
Measuring and Replacing a Suspension Spring
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Remove the Old Spring: Carefully note the orientation of blocks and fork.
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Measure: If the old spring is intact, measure its thickness with a micrometer.
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Select the Correct Replacement: Choose the exact thickness specified for the movement.
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Assemble the Suspension Unit: Attach the top block, fork, and bottom block in the correct positions.
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Fit to the Clock: Install the suspension back into the movement, ensuring the fork engages the anchor pin properly.
Fine Adjustment
- Make small adjustments once the correct spring is installed.
- Avoid using the rating nut to compensate for the wrong thickness spring - it cannot correct large timing errors.
- A properly matched spring should keep time within a few minutes per month.
Why Horolovar Springs?
The Horolovar brand is the gold standard for 400-day clock springs. Their precision manufacturing ensures exact thickness tolerances, essential for anniversary clocks. Many professional horologists will only use Horolovar for this reason.
You can read more about Horolovar on their website here