Seiko Bellmatic 4006 Mainspring (also fits 4006A, 4005, 8306A)

Genuine Generale Ressorts Swiss-made replacement mainspring for the Seiko 4006 Bell-Matic and related vintage Seiko automatic movements.

The Seiko 4006 is best known as the movement behind the mechanical Bell-Matic alarm watch. Introduced during one of Seiko’s most innovative periods, Bell-Matics remain distinctive and collectible vintage watches with an active enthusiast following.

When servicing a vintage Seiko movement, mainspring quality and dimensional consistency matter. There’s no mystery about what you’re installing: GR2378X is a branded Swiss mainspring made by Generale Ressorts, a leading specialist manufacturer of watch mainsprings.

Is GR2378X compatible with the Seiko 4006 Bell-Matic?

Yes. GR2378X is listed as a replacement automatic mainspring for Seiko/Hattori 4005, 4005A, 4006, 4006A and several related vintage Seiko calibers. Because vintage movements may have undergone previous repairs or parts substitutions, compare the existing mainspring and barrel configuration before installation.

Compatible Seiko Movements

4005
4005A
4006
4006A
830
8305B
8305C
8306A
8325A
840

Specifications

Manufacturer: Generale Ressorts
Manufacturer Part Number: GR2378X
Type: Automatic mainspring
Height: 0.95 mm
Thickness: 0.12 mm
Length: 400 mm
Barrel Diameter: 10.5 mm

Amplitude Watch Parts ships from the USA, so there’s no need to worry about long overseas shipping delays or tariffs.