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Calculate the Thickness of an In-Barrel Mainspring if you are replacing with a mainspring of a different width

If you are replacing a mainspring with a different width, then you will want to determine the thickness so as to have the same strength characteristic.

Change in Mainspring Width
Barrel and Mainspring Measurements
Inside Diameter of the Barrel
Diameter of the Barrel Arbor
Old Mainspring Thickness
Old Mainspring Length
Old Mainspring Width
New Mainspring Width
Needed Mainspring Thickness:
Needed Mainspring Length:


TMSn = ( ( LMSn X WMSo ) x TMSo3 )


LMSn = 1 x ( ( pi x DB2 ) - ( pi x DA2 ) )
2 x TMSn 4 4

  • TMSn is the new mainspring thickness
  • LMSn is the new mainspring length
  • LMSo is the old mainspring length
  • WMSn is the new mainspring width
  • WMSo is the old mainspring width
  • TMSo is the old mainspring thickness
  • LMSn is the mainspring length
  • DB is the inside diameter of the barrel
  • DA is the outside diameter of the barrel arbor

This calculator, provided by Horology - The Index, is one in a series of repair calculations.

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