A properly sized mainspring will fill half of the available area in the barrel. It is easy to calculate the dimensions of the barrel; you will need at least a fragment of the old mainspring to specify the appropriate thickness.

Calculator: Mainspring Length Barrel Measurements Inside Diameter of the Barrel Diameter of the Barrel Arbor Mainspring Thickness Results Needed Mainspring Length: Formula LMS = 1 x ( ( x DB2 ) - ( x DA2 ) ) 2TMS 4 4 where, LMS is the mainspring length TMS is the mainspring thickness DB is the inside diameter of the barrel DA is the outside diameter of the barrel arbor