# Calculating the Size of a Mainspring

Discussion in 'Hints & How-to's' started by bangster, Apr 4, 2015.

1. ### bangster Super Moderator NAWCC Member

Jan 1, 2005
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Last edited: Apr 4, 2015

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.

â€”David LaBounty

=================
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. â€”bangster

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