Introduction
Choosing the correct mainspring is essential for proper power delivery and reliable timekeeping. A mainspring that is too strong can damage the movement, while one that is too weak will cause poor performance or stoppage. This guide explains how to measure a mainspring accurately so you can order the correct replacement.
Understanding Mainspring Measurements
Width (height of the spring)
This determines how the spring fits inside the barrel or between the plates. Incorrect width causes binding or loss of power.
Thickness (strength of the spring)
Thickness determines torque. Too thick overstresses the train; too thin causes weak running.
Length (total uncoiled length)
Length affects run time and power reserve. Too long causes coil binding; too short reduces run duration.
How to Measure Mainspring Width
Use calipers for accuracy
Measure the height of the spring from edge to edge. Do not measure the barrel.
Match the original width exactly
Even small differences cause scraping or poor fit.
Check for distortion
Old springs may warp. Measure several points for consistency.
How to Measure Mainspring Thickness
Use a micrometer, not calipers
Thickness must be precise. Calipers are not accurate enough for mainspring steel.
Measure the spring body, not the ends
The ends may be worn or deformed. Measure the middle of the spring.
Match thickness exactly
Even a small increase in thickness dramatically increases torque and can damage the movement.
How to Measure Mainspring Length
Use the original spring if intact
Fully uncoil the spring carefully and measure end to end.
If the spring is broken, measure the barrel
Barrel diameter and depth can be used to calculate approximate length.
Do not guess length
Incorrect length causes binding, poor run time, or excessive stress.
Loop-End vs. Hole-End Mainsprings
Loop-end springs
Used in many American clocks. The loop must match the arbor hook size.
Hole-end springs
Used in barrel movements. The hole must match the barrel hook shape.
Do not interchange types
Loop-end and hole-end springs are not interchangeable without modification.
Common Mistakes When Measuring a Mainspring
Measuring thickness with calipers
This leads to inaccurate readings and incorrect spring selection.
Choosing a stronger spring “for more power”
Stronger springs damage pivots, gears, and plates.
Ignoring barrel fit
Springs that are too long or too wide bind inside the barrel.
Recommended Internal Links
FAQs
How do I know what mainspring my clock needs?
Measure width, thickness, and length from the original spring or match the movement’s specifications.
Can I use a thicker mainspring for more power?
No. Thicker springs overstress the movement and cause long-term damage.
What if my mainspring is broken?
You can still measure width and thickness. Length can be estimated from the barrel.
Do all clocks use the same mainspring size?
No. Sizes vary widely depending on the movement type and manufacturer.
Why is thickness the most important measurement?
Thickness determines torque. Even small changes dramatically affect performance.
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