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# Thread: Calculating the Size of a Mainspring

1. ## Calculating the Size of a Mainspring

LABOUNTY'S MAINSPRING FORMULA

The formula I use is:

L = .3927 [(D*D) - (d*d)]/T

Where:
L = length in inches
D = inside diameter of the barrel or space available for the mainspring to unwind, in inches
d = diameter of the arbor in inches
T = thickness of the mainspring in inches (not the width!)

(D*D) = D squared

With this formula you can vary the thickness of the mainspring a little (plus or minus 0.002" but favor the thinner side) and maybe find a suitable replacement.

—David LaBounty

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For open springs, let D =the diameter of the mainspring wheel. The width will be determined by the depth of the barrel or the available amount of winding arbor. —bangster

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